Will this have an effect on him as far as his own accounts. Say he wants to buy himself a new house or new vehicle, will this situation work against him in that way?
No. He is not taking a loan and nothing on his report will indicate that. However, the seller will have less money on him since he is "lending" money to you. If you purchase the house straight up or pay the seller off with a mortgage from another lender, the seller will have the money in his control and could spend it to buy something else. So no, it won't effect his credit score, but yes it will effect his ability to buy something since he will have less cash in hand than if he sold the property for money up front.
Random83 is absolutely right (and well said, R83).
However, assuming you come up with some down payment money at the time you sign a purchase contract with the seller, the seller will still be better off than he is now when you're just paying rent. Otherwise, as R83 says, this has little effect on the seller.
Also, if the current owner of the house has any experience at all in buying or selling low-end or "fixer" real estate, he'll probaly already be acquainted with seller carried financing.