Know Prices Before You Buy
Let's face it, in every economy, there are bad trades. Trading an enchanted firesword for a broken dagger is a bad trade.
The best way to avoid these in World of Warcraft is to learn prices. Where can you find out the average price for an item?
Many websites offer information, including the site at http://world of warcraft.stratics.com.
Personally, I use the addon 'auctioneer' and let it scan the Auction House contents once a day to get a feel for pricing averages, and I use WowEcon (found at http://www.world of warcraftecon.com) for hover-over quick checks for average sell prices on my server.
In short, don't buy from strangers if you're not comfortable with the pricing, but know that you may miss a great deal if you can't quickly check what's a reasonable price.