Possibly. Did you give the buyer written notice of a deadline to pay, that you'd sell the car if he missed the deadline, and that he'd be responsible for any shortfall that was beyond your control?
If not, you would likely lose in court on the issue.
In contract law, when a breach can be cured (as in this case) you must give the breaching party the opportunity to cure before you can terminate the contract.
If the breach is not cured within the specified time, you then may resell the item making sure that you have made reasonable attempt to sell for the same price. That's called mitigation. If you are compelled to sell for less due to circumstances beyond your control then the original buyer is responsible for making up the difference.
Questions:
1 - Original sale price.
2 - Did you give written notice of the breach and opportunity to cure.
3 - When you resold the car did you advertise it at the original sale price or did you advertise it for less? If less, how much and why?
The answers will give us a better idea of whether you owe the original buyer any money.
PS: I agree with Army Judge. Never take deposits. Full price in cash or no sale.