Enter into a written contract with the guy, subject to getting financing. That language must be in the contract, that if you are denied financing, the contract is dead. If you don't, the guy can sue you for breach of contract.
Once you have made such a deal with the seller, then apply for a car loan. If you have $500 or more for a down payment, your chances improve. You will not get 100% financing no matter how good your credit is.