If you've an eye for car booty, buy items cheaply at car boot sales, and sell them at a profit on eBay or other auction sites. The big money lies in spotting collectibles to sell on, so do some research on the net first. Be sure to arrive early to beat off other bargain hunters.
How much? The earning potential increases with your knowledge of rare items and collectible brands, and a little luck doesn't go amiss either; if you're in the right place at the right time this could net you £100s extra a year.