Vendors in world of warcraft will buy almost every item in the game in exchange for coin, regardless of the value another player would place on the same item. This is why you almost never vendor trash items that are white or better, since they will most likely be bought by players for a higher price than the vendor will provide.
There are times, however, when the vendor's offer will make you money.Three items in the game can often be bought off the auction house and then sold to a vendor.
Jaggal Pearl + Draenic Water -> Purified Jaggal PearlThis can be pretty amazing since people often throw jaggal pearls on the auction house for dirt cheap. If you can buy the pearls and draenic water (costs 10s88c) for less than one gold, the materials can be combined into Purified Jaggal Pearls and vendored for profit. So if you see any Jaggal Pearls for under 89s12c, you should buy them out.
Thick Spider's SilkThese lovely silks vendor for 7s50c each but are famous for being posted for less than two silver a piece.
Netherweave Cloth -> Netherweave BandagesSometimes people are so desperate to get rid of stacks of Netherweave Cloth that they don't bother to see how much the cloth is worth when turned into a bandage. You can simply buy out the cloth, make Heavy Netherweave Bandages, and sell them to a vendor for profit! If you can get the cloth cheaper than 3 gold per stack you will make a profit vendoring it as bandages.
I know that these three are not going to make you thousands of gold anytime soon, but who doesn't like free money?