Playtrade is another option. The good thing about that is that your item is on sale until sold, not just 7 or 10 days, although the fees may amount to a fair chunck for things as cheap as a CD. They charge 10% of total + 50p. (+5% to withdraw playtrade funds rather than spending them on-site)
Ebay charge 10% of item price (excl. p&p), with paypal fees being 3.4% of total + 20p.
Unless you are selling something recent, you may have to charge no more than £2 each to shift em. Ebay would be the better option. On playtrade you may make 40p after postage and fees. On Ebay, assuming you charge £1 for postage, you could make 70p for each CD.
If you tried to sell cds in bulk you'd need to make at least £7 for every 10 to be better off. That may be difficult and risky considering you'd need to either set a low starting price to avoid an insertion fee, or pay an insertion fee for a higher starting price and risk it not selling. I guess you could try selling in bulk when there is a no-insertion-fee offer on.