I've never had this request before but someone's just bought an item from me, paid and left me a note requesting feedback and that they'll return the favour.
As a seller I normally wait for them to give me feedback before leaving it for the buyer. Why would a buyer ask for FB first? This ebayer has hundreds of feedback so it can't be because he or she wants to build a good rep. It's only a small item and I see no reason to suspect anything. What do you think?