Actually, this is brilliant, not only with grafts but with magic items. Think of how difficult it might be to get the guy who wont pay for his super-killer sword, spell-caster's rod, or movement-improving (dimension door and such) amulet. Grafts are nice because you actually have to nail them down. Honestly, magic repo-men sounds like it could have a goodly number of "quests".... if I wasn't so busy, I'd totally start writing this campaign...