If a private car seller accepts a purchase deposit & sells the car before the deposit expires, can they face heavier fines than just giving ur $ back?

Fines?  No, it is not a criminal matter.  They would be responsible for the damages you suffered due to their breach of contract, so if it costs you an extra $500 to purchase a comparable car from someone else, they would be responsible to repay the deposit plus $500 (plus interest).  Some contracts do have clauses providing that if a party breaches they will have to pay "liquidated damages" in a specified amount, but such clauses are only enforceable if the damages are difficult to ascertain, and if there is some effort to relate the amount of the liquidated damages that the non-breaching party would suffer.  Neither condition sppears to apply here.   One clause which may help is a provision that the prevailing party in any lawsuit over the contract will be able to collect his reasonable attorneys' fees from the other side.