Any Advice on how to sell a Cruise Certificate?

While I have traveled quite a bit, I have taken a cruise before. A few months ago, I won a sweepstakes where I received a prize of 2 Diamond Level Certificates to Royal Caribbean good for a 5 day cruise with an Oceanview Stateroom which includes cruise fare, govt. taxes and fees as well as recommended gratuties. Travel must be completed by July 17, 2014. It's good for a Bermuda cruise leaving from Bayonne, NJ, a Western Caribbean cruise leaving from either Tampa or Ft. Lauderdale or a Northern Europe cruise leaving from Denmark, Norway or Sweden.

Anyway, after speaking with a Royal Caribbean representative, she did say that the certificates are transferable. So my question to those who have cruise experience, what is the best way and place to sell them? I would like to avoid places like craigslist for obvious reasons. Has anyone ever sold a cruise certificate before? TIA.