Selling by color
Grey: Vendor trash! Always have bag space so you will be able to carry them to a vendor.
White: If you have time, get in the habit of saving these for the AH, at least until you have figured out whether or not there’s a market for them. Anything that is AH-able is almost always worth considerably more in the AH than it is to a vendor.
Green: The advice for greens is similar, though most BoE greens, especially low-level gear, are not worth AH-ing – deposits are relatively high, especially for weapons. You’ll save time and lots of deposit fees by vendoring most greens, except for those with useful gear class (or item type) & stat combinations, namely:
- of the Eagle (Intellect+Stamina) – good for casters
- of the Sorceror(Intellect+SpellPower)
- of the Monkey (Agility+Stamina)
- of the Falcon? (Agility+Intellect)
- of the Bear (Strength+Stamina)
- of the Tiger (Strength+Agility)
These are just some examples. WoWWiki has an extensive list of item types and class preferences here. For end-game greens (i.e., 76-80 greens in WotLK), disenchanting may be the best alternative, depending on auction house prices.
Blues: BoE blues are generally worth AH-ing, particularly if they have attractive gear class / stat combinations. If they have a clear market, you may want to put them up at a nice high price at first (assuming there are no others on the AH), perhaps a handful of times, just to see if someone bites. A blue “of the Bear” item might merit a 39g/99g listing. No, it might not sell at first, but you can always re-list it. Beware doing this with weapons, however – deposits on weapons are very high, and each time the item doesn’t sell, you’ve lost your deposit.
Purple: Assuming it’s a BoE drop, and it’s for maxed out toons, the sky is the limit (or at least what others are selling it for) – make as much as you can on it, nearly always that will be by using the AH, not trade chat. BoE craftables, on the other hand, are just like most craftables. Unless they are end-game (i.e., the “best in slot” (BiS) item for a particular class), they’re just like any other crafted item: probably worth less than the mats. What everyone can do (or needs to do to level a profession), has little value.