Quark would talk a big game about using it or selling it (or both) but then just lock it up or something and pretend it never existed. Despite all appearances, he does have a conscience.
Quark's Death Note would be one good rounded episode of him creating a market of paid assasinations – before a real pro assassin who is out of contracts due to this would take it and, threating to put Quark in the list, would steal it for themselves and disappear.