Alice wants to rent a residence. She finds a house advertised and contacts the listing agent, Bob. Bob shows her the residence, and then offers her a lease agreement that requires her to pay him 3 months rent up-front (not atypical: first and last months, plus security deposit). How can she ensure that Bob has authority to lease the house? Or how can she make payment on the lease in such a way that she is protected against loss if it turns out that Bob does not have authority?
There is a notion in contract law of "apparent authority," which presumably would be satisfied if Bob can show Alice the house. But who is left with the loss if it turns out that Bob did not have authority? Just because someone has keys to a house doesn't mean they actually have authority to lease it. A fraudster who gains access to an unoccupied house could conceivably lease it to numerous parties simultaneously and then disappear with all their money. (So imagine the house is actually owned by out-of-town Zed, and Bob collects lease payments from Alice, Charlie, and Doug before disappearing.)