It is illegal in some jurisdictions, such as California, to use or attempt to use a ticket for passage in violation of its terms and conditions. Since selling or bartering FF awards violates their terms and conditions, using such a ticket is in fact illegal - not just a violation of program rules.

The violation in this case is committed by the person who uses, or tries to use, the ticket. It is not illegal per se to sell the ticket or the miles.

As far as I know, nobody has ever tried to enforce such a law in the context of sold FF awards, but there's always a first time - especially if an airline gets the idea that a well-publicized court case might be a more effective deterrent to this sort of activity than simply confiscating someone's miles.