How are the rates set for seller financing?

The interest rate on an owner-carried loan is negotiable. Ask your agent to check with a lender or mortgage broker to determine the current rate on institutional first (or second) loans.
Seller financing typically costs less than conventional financing because sellers don't charge loan fees (points). Interest rates on an owner-carried loan will also be influenced by current Treasury bill and certificate of deposit rates. Sellers usually aren't willing to carry a loan for a lower return than they would earn if their money was invested elsewhere.