The best purchase I've ever made from a charity shop was a vintage limited edition Louis Vuitton clutch bag from the Sally's Army. The zip's broken, but it doesn't matter to me and cos it's genuinely vintage (and not a modern piece with a "retro" twist) I've never seen another copy.
Other successful purchases include various pieces of jewellry, an Abercrombie and Fitch shirt and loads of pretty dresses, all at a fraction of the original price.