Don't let people tell you this isn't worth a premium vs. a standard Boxster S. Another member sold his about two years ago (MTS) and got a very good price, his was exceptional and had some very nice upgrades. His thread on selling his:
http://986forum.com/forums/general-discussions/30033-saying-goodbye-boxster-s-sold.html
As far as where to advertise your car, if it were me I'd copy and paste your ad here, Rennlist, 6speed, Renntech, Pelican and lastly Autotrader (where I found my 993). Craigslist is free and all, IMO the headaches wouldn't be worth it for this kind of a car, although the replies and trade offers can be entertaining if you are bored.