As noted above, it is unacceptable to buy a car without a clear title in hand, regardless of the sincerity of the seller.
If you like the car, give the seller a post dated cheque (he keeps the car), allowing enough time for the title to arrive.
It shows good faith on your part, and will allow him to get his act together with the reassurance he has a buyer. You can always cancel the cheque if he fails to come through, within the agreed time frame.