There is a common scam in these transactions, a subordination clause in the mortgage. This clause states that the buyer can borrow money after your mortgage is recorded and gives the new lender a mortgage superior to yours. In these clauses, you agree to subordinate your mortgage to this later loan. In other words, your first mortgage will become a second mortgage or your second mortgage will become a third mortgage. Since higher mortgages wipe out lower (junior) mortgages, you can see the danger.
I call the subordination clause a scam. It is considered a strategy by some. Seminars on how to buy property with no money sometimes teach the subordination clause as a legitimate purchase method. This is why it is so commonly used. If you are asked by a prospective buyer to insert a subordination clause into the purchase money mortgage, you can be fairly sure that this buyer intends to borrow the full value of the property with a mortgage or mortgages superior to yours. After you subordinate, you will be left with worthless paper unless the property greatly increases in value.
Subordination clauses are also used when construction is involved. This is an entirely different situation; subordinating to a construction loan may be desirable for a seller of empty land to a developer. It is not a good idea for a seller of an existing home.