I agree, OP, and whilst I understand that their main aim is to make money for their charity, it comes across as very grabby to charge what they do, for what is often, other people's cast offs.
I've also regularly seen things that are either more expensive than what you can buy new, or just a few £s cheaper, which then isn't generally worth it.
I have a friend who relied on shops like this when furnishing her new rental, and it was, in most cases, actually cheaper to buy stuff brand new from B&M, Argos, IKEA, etc.
They're definitely not there to help the needy anymore, they're just there to line their own pockets.
Quite simply, they are businesses.
I think it's wrong, but greed is the driver here. 🤷♀️