Here's at least two reasons why you might want to beware:
I know many people that have sold awards to brokers and never received the money from the broker for the awards sold. Lost in the mail, etc. until you give up. What do you do? Complain to an airline that the awards you sold weren't paid for and will they put a stop on the award use? Call the cops who in the line of investigation will talk to the airlines to establish value and remedies? I don't think so.

Also, these brokers are always living life with a rearview mirror. Airlines are constantly shutting them down and guess what the first thing the airlines do? Yep, get ahold of the customer list, both buying and selling and a little clecrk goes through the seller list and tags all those accounts. They also trade these lists with other airlines. I don't think you want to appear on any list.

So, there's a few things to be wary of. Tip: most of the deals done these days are among private parties; friend-to-friend; business associate to business associate, etc. Less of a problem and much cleaner, though "trading" is far more popular than selling.