As far as I'm concerned, the reputation function is for ANY good post you make, where someone else wants to recognize you for it.
It's not necessarily tied to buying and selling.
I'm on a couple of boards that do it the way I mentioned and it works out pretty well.
However, right now I'm not convinced it's working for everyone.
If you are logged in and have more than 1 post you should be able to see thumbs up/down for people other than yourself.