I had this happen to me after sending a 500 dollar deposit on a 220SE Ponton Cab. We sent the money by post because it was going to take a week or more to sort out shipping knowing the money would be there before the tow truck.
The same day, several hours later, he called me to say that he sold the car to someone else for 1,500 more. I said that I already sent a down payment, bought the car, and that we had a deal. He was almost whimpering on the phone saying he didn't know what to do and how confused he was about everything.
So, the next day I drove 4 hours to the guy's place and paid him in person for the higher amount because it was still a good price but higher than we first agreed to. Later on, I talked to the guy who was also looking at the car who backed off when he learned it was was me who was buying the car. I knew this guy a little bit and there was no reason not to believe him when he told me that this guy bragged to him about doing the same thing to others before me.

Yup, just another douche bag. Anyone who's done this once, has done this once before.