The Cult of Chesire Cat says:

eBay is where I sell mine.
Afternic.com and Sedo.com are also good sites to use.
However, you’ll make more money if the website has a decent Google Page Rating. Some people go specifically after sites with high PR despite what the actual name is.
ETA:
I’ve had the most success with eBay. I’ve found the key to selling it for a higher price is to stress the domain name’s turnkey possibilities. (It seems the turnkey domains go for the highest price.)
You might also get a better deal if you sell all of the domains as a single lot.
I saw a lot of 100 domains going for $1,000 a few days ago.
I sold a lot of 20 domain names on ebay for $350 with only a max PR of 2, recently.
However, you could also look at using domain brokers. who supposedly can get your domains to sell higher than on eBay. I haven’t used them myself, but you could try it for one site, see how it goes and gauge whether doing it for the remaining sites is a good idea.

Adam D says:

Big Daddy is a good on as well