As long as you're in your right mind.
It's when I start to think you're not in your right mind, or when the seller knows things about game theoretic implications of the arrangement that you don't understand (Penny Bids), that I take a dimmer view of things. And when there's compulsion or addiction, especially if it's introduced or reinforced by the seller, and the buyer doesn't really understand it . . .
Then it may not be illegal, but it's a dishonorable way to make your fortune.