Highly not recommended.

Selling awards violates the rules of every frequent flyer program. Airlines have departments with names like "revenue protection" whose only job is to catch violators. They're good at what they do. (If they didn't catch enough people to justify their cost, the airlines would get rid of them.) You're likely to find yourself somewhere with a choice of (a) staying there or (b) buying a full-fare ticket to get back. That could cost more than the original round trip would have.

Naturally, folks who want to sell you award tickets - on eBay or anywhere else - won't tell you this. Do not believe them. They want your money and are not particularly interested in what happens to you after they get it.

If it sounds too good to be true, it probably is.