If you own one or a few domains and you would like to sell them, you can put up a "for sale" ad on your blog/ site (assuming that you already have a site and are hosting it somewhere).
Or you can sign up with Sedo.
With a free Sedo member account, you can sell your domain names and manage negotiations online. And if you are not doing anything with the domain name/s, you can earn some money by parking the domain with them.
They also have over 7 million domains for sale on the site, you might find one you like.