I am a charity shop volunteer and we do our best to price at a reasonable rate. We are a small shop which is entirely volunteer run, we have no paid staff at all. We're part of a large charity you will have heard of with branches all over the country, we do get a vague pricing list but it's very open to interpretation and just classes brands as low (H&M, Tu, George) medium (M&S, Next, Fat Face) or high (Reiss, Whistles, Hobbs). We know what sells and what our customers will pay - come to our shop and tops are £5 - £6, a dress from somewhere like M&S would be £8-£9, a coat from Hobbs about £15. We are definitely seeing a decline in the quality of donations and I'm sure some people are selling their old stuff, but we still do get really good stuff.
Other charity shops are wild with their pricing, we were in North Berwick a few weeks ago (which if you don't know it is a posh wee town on the coast outside Edinburgh) and one of the chains was outrageously priced - their standard price for a dress from Next or M&S appeared to be £26.95. Crazy. But this was another large charity chain and they must be shifting stock at that price.
I also think people who don't see charity sales figures week in, week out focus solely on the clothing. Our figures show we sell more bricabrac and toys/games than we do clothing. Also the "pile it high sell it cheap" model might work for some charities but you need a lot of bodies to keep processing donations, pricing, steaming, stocking shelves, manning the till. We struggle to find the minimum of 2 people per shift.
People buy from us because when you come into the shop you can inspect the item there and then, check it for marks or other problems. Try it on and make sure it fits!! And then take it away immediately. Or if you do take it home and it doesn't fit, getting a refund is easy. Vinted is brilliant and I have bought and sold on there myself but it's a different beast.