Beware of shady auctions or bids
Look up the bidders and seller using /who to make sure they are not in the same guilds.

Auction Etiquette
Try to include a link to the item whenever possible. There are always new people showing up that may have missed the previous link.

Once people are bidding on an item, you should sell it at the end of the auction. It's rude to offer to sell an item and then retract the item from bid.

Try not to take back a bid offer unless there are special circumstances. If you have to, make sure you inform people that you are sorry that you have to retract your bid.

If someone is selling an item and you think they're asking for too much money, message them privately. They will either thank you, ignore you, or be less polite. It's rude to insult a player's asking price in public. If the item is really overpriced then no one will buy it. If someone is paying too much for an item, let them. Maybe they have a ton of money and don't care what an item is priced at, they just want the item NOW.

Don't bid on something if you don't have the money or are not ready to trade. If you're questing, you better be ready to leave and trade. You could offer to pay a tip if the person selling must wait, but don't be surprised if they resell the item.

If the seller has to wait or has to travel to deliver an item, they should be tipped over the cost of the item to compensate them for their travel time.

If someone asks for more money for an item or does anything shady then you might want to back out of the deal.