I had my car up for sale last year around Aug / Sep time and had so many muppets contacting plus serious time wasters. The money some were offering was hilarious. A few weeks back my local BMW dealer offered me £22K to buy the car outright. A good offer but I'm no longer selling my car right now.
Good luck with the sale, have you approached your local BMW dealer for a price?
Also I wouldn't bother again with Pistonheads, instead I would use Autotrader.