Buy Back Program

I was really excited to get the email saying their program to resell your old books was now up and running. I've gotta ask though...

Is anyone actually successfully selling their old books through Thriftbooks?

I'm going through my boxes of old books, checking ISBNs. I must be 20 books in, and Every Single One is coming up not available to sell, because they have too many of them already. I just find it a little weird. I've almost completely struck out. What ARE they taking?