Great business for them.
They sell $139 Kindle3 for $110 on Craiglist.
They give you $139 worth of books, on which they have plus/minus 50%. They are still 40 bucks ahead.
If it was only 10 bucks profit
- selling second-hand, never used Kindles for $80 a pop, or
- selling the Kindle for $120 on Craiglist and giving you books they purchased for $110 (20% rebate)
it is still worth the publicity.