The only way to find out what he sold it for--unless you get lucky, and just know or meet the buyer--would be to actually bring a lawsuit and use the mechanisms for finding information in a lawsuit, called "discovery," to obtain various documents (such as his "books," the sales receipt, a cancelled check or credit card statement, etc.). Of course, bringing a lawsuit can itself be an expensive endeavor, and you could easily spend more to prove what the car was sold for than you would be able to recover. At the end of the day, you agreed to a sale for $3,500--you could have said no, or put other conditions on it before approving it, like requiring that the seller furnish proof. Since instead you simply agreed to allow the sale without requiring proof in advance, as a practical proposition, there may be little or nothing you can cost-effectively do.