Selling miles would be a violation of the T&C of the mileagle program. Seeking to do so is a violation of the FT T&C.

The simple answer is "don't do it". Using eBay, Craigslist, etc is particularly risky, and there are regualr accounts of people being caught doing precisely that, and the tickets being voided and the FF accounts being closed.

Your best bet is to find a friend to whom you can "gift" a reward in exchange for a similar favor. FT does have a forum dedicated to such gifting, but it is reserved for FTers who have demonstrated a contribution to the FT community over a period of time (90 constructive posts over 90 days IIRC).