I know this answer is long after you posted the question, but two things to consider:
Amazon is very good to sell books, CD's & videos, but if selling be careful of their charges which are quite high - they take 85p plus 17% of your sale price when sold. The postage rates are fixed by Amazon. It is however free to list & of course there are a lot of visitors to the site.
If you are after more specialist items, antiques, collectibles that sort of thing, I recommend:
http://www.specialistauctions.com/
I just vistied QXL & it seems to depend what you are looking for: they have lots of books for example & almost no computer games.
Hope this helps