The seller can act as a bank and rather than receiving all or a portion of their equity at close, they can "lend" it to the buyer and receive a regular payment as agreed. They may receive no payments, interest only payments, principal only payments, or a combination. It could be an interest only loan, or an amortized loan. Additionally it could carry either a fixed rate interest payment or a variable rate. These will vary depending on the agreed upon terms of the contract between the buyer and the seller.
The seller can allow the buyer to "take over" the loan that he or she has in place. This can be done in two ways. The first way is called an "assumption", wherein the lender formally allows the buyer to assume the loan. This entails approval of the buyer's credit, and often a modification of existing loan terms. The other method is called a "subject to" where the lender is not contacted, and the buyer purchases the property "subject to" the existing financing. This can be financially risky in many ways, since many loans have acceleration clauses which permit the lender to call the loan due if the property is transferred. However, more often than not the lender will not exercise the "due on sale clause" if the payments are being made on the underlying mortgage(s). In the rare event that a lender does call the loan due then an investor could quickly sell the property or pay off the loan using any one of the various financing options available, some of which are described below.