I dont know which country you are in but If he has sold the car the new owner has the legal title to it unless the car was not yours to sell. i.e it was hired, or subject to a rental agreement, Normally only the police and similar authorities have access to VIN numbers. You may be able to find a private detective with "unofficial access to these records but even that is illegal
Once you sell an article a contract is formed and the title passes to the new buyer
Sorry I cant help any further
