Well, accoring to their website The Pricing Model is as follows:

Monthly Subscription Rates
For a reasonable [!] monthly subscription fee, which is calculated according to the amount of books you list, your books will be listed worldwide:
Number of Books        Rate per Month
0-500         £ 17.00
501-4000         £ 25.00
4001-10000         £ 28.00
10001-20000         £ 35.00
20001-30000         £ 53.00
30001-50000         £ 83.00
50001-100000         £ 116.00
100001-150000         £ 150.00
150001+         £ 200.00


Transaction Fees
Upon a successful sale via the Abebooks shopping basket, a sales commission of 8% is incurred. The maximum sales commission is £22.00 per book. When processing Visa/MasterCard payments, Abebooks will charge an additional processing fee of 5.5% of the total order value (all prices are recognised as net value).