Know your customer

Think about who is going to buy what you are selling. This will help with pricing.

Are you selling gear that someone would actually wear? How good is it?

If it's at the upper end of a level, look it up on wowhead and see if it's a twink item. If so, take the buyout listed there and add a larger percentage and ratchet it down gradually. Twink items will go for far more than comparable, but slightly inferior items. Welcome to a fundemental truth of WoW economics: min-maxers and their gold are easily separated.

If it's just decent gear; maybe "of the tiger" leather or "of the soldier" mail, then try a few times vendor price. You'll get some sales from people leveling alts who are trying to fill non-heirloom slots or who can't spare the badges. Maybe new players as well; until they learn the joys of instant loot, and then back again as they learn that the RNG hates each and every one of us, personally.

If it has terrible stats, then you're not likely to get anyone who wants to wear it. Spirit combined with physical damage stats: agi or str, is very unpopular. Those classes don't benefit much at all from spirit. With the exception of troll shamans who regenerate massive hp and no one cans top them. This sort of gear is best sold to enchanters. If you want their attention, you need to make the buyout significantly cheaper than the average DE value. Also be nice and make the bid and buyout the same. This allows instant buying without needing to confirm every item. Saves us enchanters a lot of time.

With all of these, be aware of the vendor price and the AH deposit and cut. The cut is 5%; so if vendor/.95 isn't higher than the AH price, you're losing gold; not even including the time spent and risk of the deposit. I often see people posting at prices that are so close to vendor that they lose gold by using the AH. Sometimes they even post below vendor price. Keep an eye out for those auctions!