I dont know if its the 'best' as such :S but one idea (apart from ebay anyway..) that i'd heard of is some site where you key in the numbers off the barcode and it gives you a price of what it will pay, and I might be wrong but I believe they probably pay the postage if they have them too
think its called music magpie or something. Havent actually sold any on there myself so dunno if they take the piss with their offers or not but maybe worth a look
suppose any money is better than them sitting there gathering dust and / or cluttering up the place and taking up loads of room