Walk into a car dealer, sign a sales contract, then try to back out of it once the waiting period has expired. You will quickly find out what happens!
He's legally bound by the signed sales contract to sell you the car, unless your state has a waiting period, and he backed out within the allowed time frame. If he did not exercise his option to back out during the allowed time frame, then the car is yours. I suggest you file charges with the local District Magistrate or Justice of the Peace. This can be done in the district where either of you live. In such a case, most likely, he will be ordered to turn over the vehicle and you will be ordered to turn over the balance. The court may even take the vehicle and your cash into custody, and complete the contract. In the state where I live, this will cost you under $100 and no lawyers or jury are involved as the Magistrate issues a ruling based on the facts and documentation presented. You might want to check out how it works in your state.
What you do depends on how well you know the guy. If the guy is a friend or family member, and you want to maintain the relationship, your best bet would be to get your money back and walk away from the deal. However, If you really want the car, and you don't ever care about talking to him again, it's your right to force the deal to completion. If you decide to do this, go speak to the JP ahead of time, and seek his advice, as they will usually do this if you are the party that has been wronged, which you are. Also tell him that you would like him to send the Sheriff to collect the vehicle immediately after judgement, if you win, because you feel that the guy may try to damage it in some way, hide it, or sell it to a friend in order to get out of the contract. Do not tell the seller that you are planning to file charges to force the sale. Again, discuss this with the JP, but usually the court will send him a summons to appear on the court date. If he doesn't show, you win automatically. Chances are, he will show up, and try to explain his side, but legally, he won't have a leg to stand on. Since courts will uphold a legal contract, you will win, and the deal will be forced to completion.
I know this is a little harsh, but he did it twice, and a contract is a contract. If he had any doubts, he should have never entered into a legally binding agreement. By the way, in many states, there is a clause which allows you to collect triple damages if a contract is broken. In your case, I don't know what they would base this on, but that alone may be enough to make him go through with the deal, as they would put liens against his personal property, and could even garnish his wages. Who knows; It may give you enough to buy that Hemi-Cuda!
Then again, you could take it to Judge Judy. You'd win in a heartbeat!