If the seller has a mortgage on the subject property and the person who is buying it from him does not make the monthly payment, the seller is obligated to pay.
The seller is holding all of the risk under his credit profile while hoping that the other party will pay.
The seller risks the other party destroying the property.
If the other party doesnt pay, seller will have to attempt to sell his propety again and may have to update or do repairs to make it attractive.