I agree about Oxfam. Too commercial and, for a charity shop, too expensive.
I find loads of my books in a local-to-work charity shop for our region's children's hospices (EACH shops for anyone in the East Anglia region) and I like it there because the book side is staffed by people who love and respect books, and also recognise people who go there a lot and will suggest stuff or put it to one side for you if you're a regular once they know the sort of thing you like. Which also means they attract people like us to provide them with books - not that many of mine go but I occasionally buy stuff I know I'll never read again so it might as well do something useful rather than sit gathering dust in storage where a lot of my books are as I have too many for our small flat - so the chances of finding 'gems' are good. Like the set of blue-bound, gilt-edged and -lettered 19th/v early 20th century books about myths and legends I found. Ok so that wasn't exactly a cheap impulse buy and the bank shouted at me (a lot) for it, but for a friendly, charming place like that I don't mind. In an Oxfam I'd've looked at the price and walked away.
I'm also very lucky that there's a chap on Norwich market who runs a 2nd hand bookstall, he's also obviously someone who loves books and really values his customers - once he knows you he'll look out for stuff, round your prices down to the nearest £5 and so on and I'd much rather support people like him than any commercial outlet.
I like 2nd hand books better usually anyway, apart from the obvious not-bankrupting-myself-is-good appeal, I think books should be a bit worn and battered, it shows they've had an interesting life and been well-read and loved. And dropped in the bath, chewed by horses, slept on by cats, hauled about everywhere in bags or stuffed under motorbike seats, and other such abuse that mine get. Which could be seen as me not taking care of them and giving them the respect a book deserves, or that I love them and they go everywhere with me as I won't be parted from them - see it as you will! There's only a few people I'll lend my books to, namely the ones I know will give them back, the exception being my copy of Prozac Nation - that book can be a lifeline in bad times and I've lost 2 or 3 copies lending them to people. They just vanish and neither of us can ever work out where to, so my copy now doesn't leave my shelf (unless it's with me), if someone picks it up and reads a bit I'll buy them their own rather than allow them to borrow mine since the time I lost my heavily-annotated copy.
*end of my two-penn'orth*