Owner financing may not affect your credit in the same way a traditional mortgage might, but it can still have an effect.
For example, if the seller decides not to report your payments to the credit bureaus, paying on time won't help build your credit score and missing a payment here or there won't harm it either. However, if you decide to stop making payments on your mortgage and default on the loan, the seller can take you to court and file a judgment against you, which will negatively impact your score.