I tested uShop, and it's a bug-ridden pile of crap.
I tried Yoshy's store hack, but it's still in beta and needs a bit of clean-up work, and he hasn't updated it in over a year. If I don't find anything else, I could probably fix it up myself, but it'll take a while.

Does anyone know of a good, stable, finished, free shop hack for vB3?