There's Amazon and I think that may be the best for prices since it's going to be what folk are looking for and you can see what others are selling the title for. Otherwise you have Ebay and sadly I see such usually for a dollar each at garage sales.
Where are you? In the UK we have markets like Music Magpie that will give you quotes for items in your collection. Failing that, you can always sell through eBay or Amazon but if you have lots of CDs, creating lots of ads might be a tedius prospect.
Be warned though, the likes of Music Magpie (and the others) pay pennies for such things. I had a bunch of DVDs to sell, and was offered prices as low as 50p for a Big Bang Theory multi-series boxset FFS! and 10p for individual items. I would imagine the prices for CDs are even lower. I ended up giving them to a charity shop rather than letting some commercial outfit sell them on at a quid or two each time.