Since feedback is voluntary, and only about 70% of transactions end with feedback, there really isn't any particular reason for a buyer to worry about it.
Feedback left for a buyer is egoboo, which is nice to have, but not meaningful, since it must always be positive.
Feedback left for a seller who wins a case -- well it can't be left, since revenge might come into things. Feedback for a seller who loses as case-- gloating.
Not the best view of human nature, really.
If the other person has left feedback, you can add a Response to it. If the feedback comes under certain restrictions it can be removed.
But if all you want to say to the other person is 'thank you' , send an eBay message. The kind thought is there.