Once every few months, there are promotions that when you
transfer your AMEX points to certain programs, you can get
a 30-50% bonus. Dansdeals always informs of those deals.
The best programs to transfer points to, are Aeroplan and ANA,
and you can make business class tickets around the world.
Selling / bartering your points goes against the terms and conditions of the AMEX points program, and all airline programs.