Whether you buy a preowned or a new vehicle is not important to the credit score. The credit scoring model is only concerned with the credit information that is being reported for the consumer. So if you take an automobile loan to buy a vehicle, the credit scoring model will only consider the amount of the loan and your regularity in making the payments. Whether the loan has been taken for a new vehicle for a preowned vehicle will make no difference in the calculation of the credit score.
However, the only snag that may occur while buying a pre-owned vehicle, in some instances, the financial companies do not report your account to the National credit bureaus. In this case the loan that you take from your creditor to buy the preowned vehicle will do nothing to improve your credit score. If it is important to you that the loan should be reported to the credit bureau and enhance your credit history then you should ask the lender whether or not he reports the loans for pre owned vehicles to the credit bureaus.