I've bought items only on games like Kingdom of Loathing [http://www5.kingdomofloathing.com/], which are free to begin with and subsist entirely on donation. I see little point on doing so anywhere else. If the bought items can also be acquired through gameplay then I shall acquire then through gameplay. If not, it is a bad game and I don't want to play it. (Likewise, if the bought items can be acquired through gameplay but it's extremely boring, it's also a bad game and I don't want to play it either.)