I personally would not do/recommend this. In the example you gave (and most if not all FFPs), AA explicitly prohibited the sales/purchases/barter of miles. When one gets caught, which is not at all uncommon, the buyer will have to pay full fare at the gate to get onboard. The seller's AA account will be frozen and a hefty fine will be charged to un-freeze the account, if at all possible. Search and you will find a lot of sorry stories on this on FT. Do at your own risk is what I would say.