My experience is that they are typically charities. You go in just like it is a bike shop. Prices are sometimes a bit higher than private sales, but the money goes to good causes. WorkingBikes, in Chicago, ships excess bikes to countries where they can be put to good use. I happily paid a bit more for a bike for my wife to ride. I will be unhappily selling it or giving it away soon, because she can no longer ride and never did.