Personally I agree with what Peasebottom has already stated. However, if you are willing to sell the vehicle to the buyer but are running out of patience with him/her, then send the buyer a message saying somethng along the lines of "I am only willing to wait until [insert date here] at the latest for you to collect the vehicle and pay the remander of the balance. If the vehicle has not been collected by this date and the remainder of the balance paid in full then I will be returning your deposit in its entirety, as I have already been waiting for four months for you to collect the vehicle."
Although some people may feel that this kind of response is a bit blunt, it will at least let the buyer know where he/she stands, as well as delivering a "Stop messing me around" message, and perhaps will even serve to give him/her the necessary kick up the backside to pick the vehicle up in the near future so that you can get the sale completed.
In the event that the buyer objects to your message regarding the vehicle refund the deposit in full as per your warning, add the buyer's User ID to your Blocked Bidders List and keep the relisted version of the ad live so as to maximise your chances of attracting a buyer who won't mess you around and will collect the vehicle within an acceptable timeframe and pay you the money in full.
