I had done a bit of day sailing, completed three classes with U.S. Sailing and a charter in the Grenadines before responding to my first crew wanted post. I wanted to insure that I was at least a minimal asset rather than a liability. An email followed up with a phone call landed me the ride, which I've since used as a springboard for several other offshore deliveries.
One USCG licensed master I've sailed with said his success rate with website-recruited crew members has been about one in ten -- I assume I'm in the minority since I've now made two 1,500 NM deliveries with him. I don't exaggerate my experience, make it clear up front that I'm there to learn and subscribe to the theory of "Your boat, your rules."
Posting "crew available" notices, however, doesn't seem to work very well unless one can start the post off with the something like, "Large-breasted female with low moral standards seeks..."