Every oxfam I've been in refuses to sell, giving the same answer of "goes to head office".
British Heart Foundation will sell you anything they have at a reasonable price.
When the staff say they have no idea of value they mean it, but I'll always give them what I think is fair + some for charity too. - I often pay 3 - 5 x what they initially ask locally, as otherwise I'd be taking the piss.
Sometimes in the job lots I get really lucky, but normally I'll break even taking that gamble.