Afternic and Sedo are naturally the big guns for listing your domains,
Sedo is free to list and only takes a commission when you accept an offer on a domain.
Afternic have a yearly membership fee of $19.95 as well as taking a commission on the final selling price.
tdnam is another good place to list
Bare in mind though exposure at these will be minimal due to the huge catalogue of domains listed there (4 million names now on sedo) so for quick sales this isn't much good. Out of the two I've had more success with Sedo.
For quicker sales I use forums looking for people who post wanted ads, or indeed post them for quick sales.
eBay is another option but you will rarely receive end-user bids. It will be mostly other domain speculators trying to pick up bargain domains to add to their portfolio.
Good Luck