I think the key here is knowing the property is stolen. I can imagine that pawn shops and thrift stores receive items that weren't exactly donated voluntarily. But they aren't going to be charged with receiving stolen property if they can plead ignorance. There wasn't anyone around who could verify the ownership of those items, or the shop owners were duped by sellers under false pretenses.
I think the minute you suspect something might be stolen, you should just walk away from the deal. I've been offered great deals on everything from televisions to used cars, and most of the time they turned out to be stolen. All a police officer has to do is recognize the item on my porch and I'll be charged with possession of stolen property.