You can't really have it both ways though... the real bargains for the customer were the loss for the charity shop in their own way.
I don't entirely agree, as you get no sort of guarantee with charity shop finds, so the price has to be lower than other sources, I would say. Certainly, I'd never dream of taking faulty goods back to a charity shop, but I'm prepared to take the chance if it's in a good cause.
That said, I'd never haggle in a charity shop, either - I've seen people try it, and I think that's pretty low (unless an object is clearly marked way too high, such as the LPs previously mentioned, in which case a polite word to the manger would be in order).
As for bargains, I've found boxes of Lego* (a guilty pleasure of mine) at silly prices, as well as model-railway related stuff, which I'll take a punt on, reliability-wise, if the price is low enough. I try to tell myself that both parties benefit from such bargains - after all, the shop haven't had to pay for the stock, and it's unlikely they have to pay much in the way of wages, either, so profit margins must be OK.
I recall a volunteer once spotting a slightly beatup bronze colour Quad ESL57 which almost went in the trash, as they had a tendency to be mistaken for space heaters.
Ah, the old ESL57, good old Quad ESL57, old Quaddy-Quaddy... what's that, then?
*Edited to add - Lego has to be proper Lego. None of your Megablocks nonsense, or worse.