Its really not against the law. Its a breach of your agreement with the airline and gives them the right to impose a forfeiture of your other miles (according to the airlines, although the legality of this is questionable). Its also a violation of the TOS of eBay because they don't want to get sued by the airline for interfering with their contract with their FF's. So certain consequences may result, but its not against the law in the sense that you are a criminal or can go to jail.