bmwcca.org, if you're a member. I had over 200 responses and thousands of views. You can do the website ad, Roundel ad, or both. Unlike Criagslist and Autotrader, everybody's looking for a BMW, so the number of lowball-flippers-to-be are far fewer.
Unlike Craigslist, there's less chance of a serial killer showing up at your door to look at the car, and less chance of you ended up in the trunk dead. BMWCCA has very few members who are serial killers (except maybe that crazy **** in Santa Barbra). Although, being a convertible also diminishes the chances of you ending up dead in the trunk... because of the smaller trunk.