Simple, just like shrine dust, Vendor Dust (low end) turns any vendor into a 80% buy-back vendor (or the tool vendor turns 90%), and the super-powered version turns any vendor into 90% (or the tool vendor into 100%)
I like the idea a lot, as long as it were well balanced and not something only the rich could afford to make themselves richer. I can't think of a way to make it really balanced out, but that doesn't mean it would be impossible to do so.