Closing out an account and selling the miles seems pretty low risk. The money is in your bank account before the award flight is taken. If not just call and complain / cancel award reservation.
But who in the WORLD would buy a ticket from these "brokers" - it seems like so much can go wrong. The original seller reneges, the broker goes back on the deal, the airline cancels the tickets out - then you are left with a wrecked trip which has got to be super expensive to fix.
So I have no idea how these businesses stay in business unless some of them have really worked it out so the flyer is OK.