Well, it's been quite a while since my first year of law school, but the elements of a legally binding contract are offer, acceptance, consideration, and a meeting of the minds (aka mutuality of obligation). Here, a contract was formed when the seller offered to sell a car for $8500 and the buyer accepted that offer and tendered payment in the amount of $8500. However, the seller is currently in breach of his duty under the contract to deliver the car to the buyer. That would be the basis for OP's suit and his damages.
But the real point here is, the OP just wants his car and parts back. A small claims suit would be mostly to get the seller's attention. Some 90% of such suits are settled before they ever get to court, either by the parties directly, or court-sponsored mediation.