A seller could finance and take a mortgage for the total purchase price. Now the transaction is strictly between the seller and the buyer. There is no first mortgage lender to qualify the buyer or to deal with in a foreclosure. It is all up to the seller.
In some parts of the country, a seller customarily uses an attorney to facilitate the sale. If you live in a state where lawyers are used for home sales, be sure that you consult yours before signing anything obligating you to receive a mortgage as all or part of the purchase price. If you live in a state where attorneys are not customarily used for home sales, you should consult one anyway. Some state bar associations certify lawyers as experts in real estate transactions. Call your local bar association first. If your state bar cannot give you the name of a qualified attorney, a recommendation from your bank or title company can usually help.
Have the attorney either draw the note and mortgage or examine the one that you plan to use. This is especially important if you plan to sell the mortgage. You must be sure that your note is a negotiable instrument. If not, you will have difficulty selling it. There are whole courses in law school on negotiable instruments, but the important thing to know is that the documents must be drawn (written) properly to be negotiable, and a buyer of the note and mortgage will want negotiability.