I LOVE thrift store play dates. Even when a really nice and well-meaning lady comes up to me while I sit with my baby in the toy aisle, hands me a simple baby board book, looks at our pile of toys and says "you should consider this too. Trust me, I'm a teacher." (She probably doesn't know that our apartment is a fire hazard for all of the books inside.)
Usually though, everyone just assumes that everyone else is doing the best they can for their children, and the more people come, the more it is clear that we're all in good company.