Don't know about miles, but if you look at the UA SWU's, they are being offered on ebay by two distinctly different groups.

Group 1: Has sold 100-1000 items, and tend to have a lot of SWU's available often. None their SWU's so how do you ding them?

Group 2: Sold nothing else. Probably their own SWU, how do you prove that the seller EASTEREGGHEAD is really Ms. Jo. Blow, unless UA buys them and traces the money?

Selling miles? I think one can buy miles for $0.03, so unless one is selling 50 or 100k, why bother, and the buyer has the big problem, anyhow.