Here's a cautionary tale-- pay the airlines for their FF miles if you must, but don't pay anyone else.
Airlines have officially forbidden the sale of airline miles, but apparently the practice has been flourishing since the principle of free commerce has been upheld in court. However being right is not the same as being able to do it.
A "friend" of ours sold 60K of her miles, her entire account balance, to a Craigslist advertiser for a healthy sum (the airlines typically value their miles at about two cents). The two of them went up to the ticket agent, she declared her desire to transfer miles to this guy's account, and the ticket agent did so without any questions. Her account was duly zeroed out and when they were outside the building the guy handed her $500, which presumably accounted for about half of the money he'd just saved.
Later the guy said that he did this with at least one other person who was much more nervous chatty. This seller asked the agent a lot of questions that flagged some official to conclude that the guy was buying. The transaction was completed without comment but the airline had the last laugh.
When the guy was sitting at the gate waiting for his turn to board the aircraft, he was called to the counter. As the other passengers boarded, a couple of big airline employees in dark suits somberly shared their opinion that he had improperly purchased airline miles in a manner not permitted by their rules and was thus holding an invalid ticket. Said ticket could be made valid by immediately reimbursing the airline $900. The guy was free to dispute their opinion and appeal it with the airline or other authorities but regrettably he would be unable to board the flight with his invalid ticket. He decided that at that particular moment a valid ticket was worth the $900 (plus the $500 and whatever he'd paid the other seller).
A few days after the guy called to share his tale of woe, the airline sent her a letter professing their belief that she had inappropriately sold her miles. To remediate the transgression an additional 60K miles had been deducted from her account. She was free to dispute their opinion and blah blah blah...
So I doubt she'll ever fly with THAT airline again, presuming they survive bankruptcy! In an unexpected benefit, they finally stopped sending their unsolicited offers from their valued affiliates.