This is a common practice in the collector car world as well. Goes sorta like this.
A person will come out and look at a car you are selling. They will strike a deal with you and will give you a small deposit and set a completion date of two weeks. They complete a bill of sale showing the deposit paid and balance owing by a certain date (2 weeks from today). They gain legal claim to your vehicle.
They then list the vehicle for sale for more money on e-Bay or locally. If they sell the vehicle they come and complete the deal with you and flip your vehicle and turn a profit with minimal investment and risk.
If they are unable to sell the vehicle they call you and tell you they are unable to complete the transaction due to an unforeseeable misfortune. If you refuse to buy into their sad bad luck story, then they will let you keep the deposit.
This lowlife obviously is doing it without investing any money or having a legal claim to the signs before listing them.
Dave's Garage & Memorabilia, Inc.