In aid of the Starlight Children's Foundation charity, of which he is Ambassador, Matt is auctioning off his first ever car, which still sits on the drive outside his parents' house.
Details can be found on the auction:
http://www.ebay.co.uk/itm/first-ever-car-Dr-Whos-Matt-Smith-Vauxhall-Corsa-aka-Shed-/151078388808 There are only five days left, so if you want so ride around in the same seat in which dat ass has been sitting, then better start selling your own stuff or relatives soon...