I’ve set things up so that whenever I review something which I have spare copies of on hand, you'll see a PayPal "buy now" button on the page for that item. I've started doing this because I've noticed that some of the things I've reviewed have been extremely rare, and people often ask "Where do I get one of those?" when I just so happen to have a spare copy.
Check out Tokyo Flashback 2: P.S.F. Psychedelic Sampler and Rays of Beauty (C.M. von Hausswolff) for examples of how this is being implemented. I’ll eventually have a way to browse all of the items for sale on a single page, too.