recently i've been trying to throw a bit o' cash around charity shops rather than buying stuff of Amazon, etc (books, dvds, music, never EVER clothes, obviously), but they're generally shit.
firstly, people are just on to the good stuff nowadays. you might find the odd good book, but music-wise, nah. they keep the best stuff, often to sell online, and the professional penny-pinchers do their daily rounds for the rest.
also, their prices aren't really realistic. for example, penguin book, or wordsworth books, y'know, the cheap kind, they're more expensive to buy old, browning and dusty than they are new. not payin' it.
anyway, what good things do you find in charity shops, and what sections do you avoid completely, assuming that you don't swerve all second-hand places, as many do?