Not sure but its not much, I sold some textbooks on there and after fees it was still worth it to sell. They also pay for shipping in advance as far as I can tell.
I just pulled up an old inventory receipt from my half...I sold a book for $62, they took a $7.75 commission and reimbursed me $2.64 or so for shipping.
If it's academic books/textbooks etc you're interested in, I have had a lot of success with academicbooktrade.co.uk and it is completely free for unlimited listing.