I have found some bargains in UK charity shops, but it sounds as though the USA stores may carry a better selection.
Many UK charities are anti-knife and wont sell multi tools or SAKs and the like. And even if the charity has no such policy, individual staff might.
There is also a general reluctance to handle "hazardous" goods including camp stoves or fuel burning lanterns.

I have obtained several pocket knives and multi tools "via the back door" of a local charity shop. I know well one of the volunteers at the shop and they save for me certain prohibited items that would never appear in the shop.