In my opinion, you're better off selling the diamond locally through the classifieds, or through a jeweler on consignment.
A good appraisal will go a long way towards helping you sell it.
On-line selling is much riskier (in general), and you usually get less because of the intense competition.
MidWest Gems has a great page about selling your jewelry at:
http://www.midwestgems.com/sell20.html