Etiquette

I find it best to be proper in response to all reasonable requests (note the word "reasonable"), but to constantly push for self-reliance: there are few cases (like being in danger of losing an irreplaceable everything) when another player really needs the help of another. (The usual solution is to gain a few levels, save up some gold, go hunting for equipment or whatnot.) While multiplayer is encouraged, a character should be able to finish an act (but not necessarily the boss) by herself before moving on to the next. Otherwise, she's just not ready for that act. Hence, a character who is being thrashed in A3 should move back to A2 to gain levels (preferably while moving through the entire act) and prepare themselves for A3, not ask for the help of another (unless it's Hell difficulty, where everything is hard).

This being said, one should exercise proper conduct at all times, even on b.net. Especially on b.net, since words — and the way a person presents those words — mean everything: there is no body language, no effective ways to communicate sarcasm and no facial expressions to follow. People tend to respond positively towards a polite person and the attitude one displays under the cover of anonymity is, more often than not, what that person is truly like. Metal is tested by fire; man is tested by his words.

Don't beg for gear, ever. Trade, call in a favor from a friend or be receptive to the occasional high-level who likes to give away stuff (but don't make it a habit. Those high-level players do it to be nice, not because they're obligated to help.) Don't ask anyone to help you kill a boss for you, ever. If you can't get past a boss, then gain some levels, get some gear: the bosses are what cull the weak, impatient and stupid from the smart, persistent and strong.

CAPITAL LETTERS ARE ANNOYING AND HARD TO READ. Unless you're Hunter S. Thompson, use them sparingly; type was designed to be set in both upper- and lowercase with ascenders and descenders so they'd be easier to read. It has been that way for hundreds of years; don't expect to break this rule anytime soon. Those who use capital letters may be too stupid to type properly, overly excitable or too inconsiderate to find the CAPS LOCK key, forgetting that capital letters EQUATE TO SHOUTING ON THE INTERNET.

Dueling Etiquette: Player versus Player

Though I can see to the ends of the earth
Still I ramble this treacherous path.
Over my shoulder I glimpse the corpses gathering.
This day I walk the Way of the Beast.

— Hiroaki Samura, trans.

PvP is not my favorite activity; refer to PvME's Bowazon Dueling Guide and ChunLi's Assassin Compendium: as references on dueling as a bowazon. The advice I give to prospective duelists is; one, don't go hostile on anyone for no reason at all. Be a duelist, not a player-killer. Potions (of any kind) are not allowed in duels. The Buriza-Do Kyanon ballista is generally not accepted in duels: it's too easy to fire off one or two Guided Arrows and kill a target with 100% piercing. Overloading on items with PDR is generally not accepted into duels, as Shaftstop, Vampire Gaze and String of Ears give the wearer a 55-70% resistance to physical attacks, in addition to the 50% damage penalty of PvP, making anyone who wears PDR items incredibly hard to kill. Slow Target items like Cleglaw's Pincers or Nosferatu Coil are generally not accepted in duels as Cleglaw's Pincers chain gloves alone can render any target helpless and having more than 40% Slows Target simply makes things too easy for a bowazon who doesn't even need to look at her target to kill it. The same applies to items with knockback and items that cast Amplify Damage. Keep in mind that all damage is cut and calculate your equipment accordingly. Try to keep it fair and note that clvl-1-29 is its own special area, then 30 on up; it's clearly not fair for a clvl-31 character to duel a clvl-28 character, much less a clvl-45 to take on a clvl-20.

Remember, if your opponent runs back to town, you've won: if they weren't in danger of dying right then and there, they wouldn't go to town, now would they?

Trading

Want to avoid getting cheated? Then avoid the so-called "trading channels." The thing that is most accepted and demanded (begged for?) at trading channels are Stone of Jordan rings. If you don't want to trade SoJs, then find a game called "trade#" or "trades#", where "#" is anywhere from 1-20. Most of these games are normal, with a bunch of Meph runners who don't have tons of SoJ and simply want items. Surprisingly, you can get a ton of good deals here, so long as you don't base them on a SoJ-to-SoJ basis. Another place to go would be the marketplace at diabloii.net, where scams and bad deals are easier to avoid due to valuation forums and feedback.

Now for some particulars…

The softcore economy revolves around two things: player-vs-player and magic find. These are the two most important things in softcore; thus, items with high magic find or immediate use in PvP are the most valuable. Consequently, items with little use for magic find or PvP will cost relatively little, even if they're more valuable for everything else. The "currency" of softcore is the Stone of Jordan rings and the following tips mostly apply to softcore.
The hardcore economy revolves around staying alive: any game a character can walk away from is a good one. Things like MF and PvP are cute, but nowhere near as important as vita and resistance charms or items with massive life bonuses. Presumably, the "currency" of HC is the +19-20 life small charm; because charms cannot be looted from a dead HC character and items are so easy to lose, things tend to be cheaper in HC. For those just coming from softcore, SoJs mean relatively little in HC, as they have no resists, no bonuses to life and no bonuses to any attributes, making them very iffy in the overall picture of survival.
No normal unique or set item (or even a set), save a SoJ, is worth a SoJ. In fact, with a few exceptions (a perfect set of Chance Guards chain gloves, a perfect Nagelring), no amount of normal uniques can get you a SoJ. If you want to trade normal ready-mades, use chipped/perfect gems or runes used for crafting, which can then be used to "buy" a SoJ.
For the most part, no exceptional unique or set item (with a few exceptions) is worth a SoJ by themselves, even if they're good enough to be worth a SoJ. Again, stones are used to pay for MF, PvP and end-game equipment. Elite uniques/set items are usually highly valued because they definitely qualify as end-game equipment; there are a few exceptions, but for the most part, most elite items will cost at least one SoJ.
Rares usually don't trade well, except for rare jewelry, circlets and the better-than-godly CD2 pre-v.07 rares. Keep in mind that the circlets that're usually the lowest in demand are for druids and barbarians, because of Jalal's Mane, Arreat's Face and Vampire Gaze. In fact, circlets are iffy affairs all over, because there is a piece of ready-made gear that more or less suits a class's needs quite well on its own…like Harlequin's Crest. Amulets are tricky affair as well, since a decent Mara's can usually fulfill the +2 skills and prismatic right off the bat, so you want to start looking for other things like fast cast, mana leech, MF% in addition to +skills and prismatic. Rings…well, rings are usually self-explanatory things, since they're the only thing Blizzard hasn't made an overwhelmingly powerful ready-made of yet.
Cruel weaponry goes for a surprising amount nowadays, as socketables allow for a lot of customization; due to sockets, cruel weapons are also one-of-a-kind items to some extent. One empty socket is nice, but two empty sockets can literally triple a good cruel's value. Cruel swords with 3 sockets are a generally good item to trade. However, when it comes to cruel weaponry, always, always, always list the +ED%: the closer it comes to 300%, the better. A good starting point for me would be 260% (just a bit stronger than Grandmaster's). Before trading for a cruel, find out if it has a suffix (and what it is) and how many empty sockets it has (if any). The price difference between a plain +300% cruel matriarchal bow and a +275% cruel matriarchal bow of alacrity with two empty sockets is vast. (Actually, a +300% cruel matriarchal bow of alacrity with two empty sockets can get you just about anything in the game, including a Windforce hydra bow with change. Why? PvP, baby.)
Keep an eye on those variable mods. Because ready-mades are always available, you want that item to be the best sample you can possibly get, so when you have an item in mind, look up the variation at the Arreat Summit, note an acceptable range of percentages, then get to work. Note that while perfection is nice, perfection isn't worth that much more. Likewise, the enhanced defense ranges on armor and shields is totally inconsequential: just get what you can. For the most part, anything within 10-20% of the maximum is perfectly acceptable.
Accept runes. By God, take runes: most are used in components in Horadric recipes or rune words. I've often found people are more willing to give up Um runes than they are stones and I go through Um runes like nobody's business.
Do take item-for-item trades, especially if it qualifies as end-game equipment. Even if you don't need an item, someone else out there might.
Traders tend to keep their uniques unidentified on the pretense that unidentified equipment cannot be duped. Consequently, unidentified equipment usually goes for more. Don't fall for the hype: unidentified equipment can and has been duped. Because most ready-mades have variable attributes, you could find yourself trading for a "low" version of whatever it is you're trading for. Also, consider that out of all the ready-mades available that can be considered end-game equipment, very few pieces can be considered "dupe-worthy," and for the most part, they're all elite. The only three exceptional uniques that can be considered "dupe-worthy" are the v1.08 versions of the Saracen's Chance, Vampire Gaze, Valkyrie Wing and Gerke's Sanctuary. The rule of thumb is, if Mephisto, Pindleskin or a cow have a very good chance of dropping the item you've been looking for, then don't bother looking for an unidentified version. You'll save yourself quite a bit of trading revenue this way.