Personally, I wouldn't take the chance on eBay or anyplace else where you don't know the person in some way. Ethics and legalities aside, too much chance of getting busted. The seller may be under airline scrutiny already - they watch these things, you know.

This is from someone who (a) buys and sells stuff on eBay, and feels pretty good about it; and (b) has swapped upgrades with FT friends, and feels OK with that too.

YMMV.