The airlines don't like it. It violates their programs' rules. They dislike it so much that they have groups of people dedicated to stopping it. These people are good at their jobs. The consequences if you're caught, as posted, will be:

> To the seller: Closure of account, forfeiture of all points/status.

> To the buyer: Cancellation of ticket at the airport, leaving a choice between paying the full last-minute fare or staying on the ground.

The airlines' revenue protection units (names vary) don't catch everyone. They may not even catch a majority of the violators. However, they catch a lot of people. Thanks to the magic of "big data" and data mining, they get better all the time. I wouldn't try it. Since you came here for advice, I advise you not try it either.

(It has been suggested in other threads that selling miles/award tickets is actually illegal in the strict sense of the word, since in at least some places it is a violation of the law to use a ticket for transportation in violation of its terms and conditions. I am not a lawyer, I doubt anyone has been hauled into court for this, but it sounds plausible to me.)