AfterNIC is OK, but overzealous domain owners try to inflate appraisals of their names.
eBay is OK to, but people mess up the integrity of the site by asking insane prices for their domains.
The important thing to know is that Network Solutions will charge the new owner for registration as well. Also, did you hear about races.com? This guy bought it and started the tranfer process with NetWork solutions. NetSol botched it up, and the guy lost the domain. Just be careful.