I think, no, I know, that the days of a charity shop being a source of rare records are long gone; they usually employ or have contact with a valuer so I wouldn't expect to find anything of any real value that has been unintentionally or unwittingly underpriced. You'll get your best-sellers at a reasonable price but they are acutely aware of just what pink Island, blue :Liberty, Greensleeves and Blue-Note, for example, mean in terms of collectibility: they use the Record Collector 'bible' and often grade slightly higher than I would. I know one of these valuers and I've seen him, on many occasions, 'in action', trying to get stuff on the cheap from a market trader to sell at a charity shop; it used to be, and still often is, the other way around where a market trader or dealer will look to pick up something of value from a charity shop, but, as I said, they now have an acute awareness of the market.
Better by far to find a market with 'a record man' and get to know him/her and establish a relationship which will lead to you getting favourable deals and this works both ways of course because you'll keep going back. It's also better than buying stuff online because you can see what you're getting and, if the record has a fault, ie a sharp warp or a scratch that stops the stylus in its tracks, you can take it back and get a refund next week. Well, that's my experience. I like to haggle, argue and barter and I'm quite prepared to walk away with a VG copy of a rare record at a knock down price and clean it up: I don't care about perfection if it's something I want: I'm a collector first and a hifi freak second!
We're all junkies; we find our fix where we can