Traveling Merchants
Some merchants make their rounds around the regions in the world. If you see a traveling salesman, stop them and see what they have to sell. It might be very rare!

Recipes/Training
Some merchants sell rare recipes or trade skill training. Be on the lookout for them.

Buyback
If you accidentally sell an item you had not intended, you can buy it back for the same price at which it was sold. You can repurchase up to 12 items that you have sold to a vendor. There is a buyback tab on all merchants that you can select to repurchase the last 12 items you sold since you last logged in. When your character leaves the game or zones to a different area, this list of items will be cleared, so if you have accidentally sold something, don't wait to buy it back. Buy it back immediately.