I can't wait until the boot fair season starts. People are selling so many rare games and they don't even know it.
Best deal would have had to have been Final Fantasy VII (black label) for 20p at a boot fair, although it was my step brother that found it so that doesn't really count.
I've had quite a few good deals off my friends as well. One friend gave me his copy of Final Fantasy VII, another gave me his Nintendo DS and I recently bought my friends Vita, protective case, 8gb memory card, Uncharted and LBP for £120.