It's overall quite true. But I found that the smaller stores, that live in the shadows of a larger one, do still offer the actual bargains. For example, in Emmaüs Tarare, a smaller city 30 min away from Lyon, you still find solid furniture that can be up to 4 or 5 times cheaper than flimsier pieces in Emmaüs Lyon.
I don't quite know what the dynamics are, here.
I guess it's the same for other charities/thrift shops/second-hand shops worldwide. I've seen the same with Stockholms Stadsmisssion in Sweden: more centrally located stores have nothing of note, more remote ones and smaller ones may have actual treasures.
Your best bet is a less dense but affluent area. The general level of income near thrift shops is probably the best proxy to whether or not good deals will be there across the board.
