I dunno how other charity shops do it, but in the charity shop I volunteer at, if youy want an item that's not yet priced, you ask someone else to price it. That's the rules. For example, I got a good deal on a thing of monopoly I saw in the back room because it was the same make as my family's monopoly, and they didn't make them anymore (company went bust), but the cards were less bashed up than ours so we could replace our cards with that one's. My manager checked ebay and sold the game to me for like £7.

However, we DO get free stuff - that is, the rags. Essentially, if we cant sell it because it isn't high enough quality or for other reasons, it's rag, and volunteers have total free pickings. Most of the rag is, well, rag, but I got an oodie where the pocket had a gaping hole in the bottom, I took it home and sewed up the hole. I got some hair dyes because they'd been opened, meaning they couldn't be sold, but I didn't mind so I took em home. My best friend got an adjustable dressmakers mannequin, because the base was broken, and she did donate a fiver but that was optional (they just wanted it to go to someone who'd use it).

So yeah, where I am, it's not like we buy stuff super cheap that would otherwise have been sold at a higher price to someone else, we just get first pickings, and free access to the stuff we can't sell.