Think outside the proverbial box as well, the weather has been awful, no respite from it, who is looking to buy a rear wheel drive sports car at the moment? There could be a multitude of things going on which are putting people off as well as your ad.
Sounds daft, but have you tried sticking a For Sale sign in the window? My dad just sold his BMW in a day doing exactly that, no premium cost postings on PH or anything, just selling it the old fashioned way