In order to pawn an item (in most places) you must produce an ID of some kind (drivers license, etc.) and sign a certification that you are, indeed, the owner of the item. It is not difficult to get a fake ID in order to comply with the certification. Sure, sometimes people aren't entirely truthful about how they acquired an item. But a pawn broker has little choice but to believe that what is certified is true. But most of them aren't stupid. They know it is against the law to receive stolen goods and so, if they have suspicions about a transaction, they will decline to accept the goods.

Pawn brokers are businessmen. They do what they can to stay in business. Most try to maintain a good, clean operation. But they are people. Sometimes they make mistakes. Sometimes some of them even make those mistakes on purpose.