I haven’t personally been involved in any seller carryback mortgages.
I associate seller carryback financing with high interest rates. That is, when interest rates are high, sellers may have to finance the sale themselves to get the property sold.
Bob Bruss (see article linked) tauts them as a good option for some retirees when they downsize to a retirement home.
With a seller carryback, the retiring seller can receive a stream of income at a good interest rate instead of a lump sum.
In addition, the seller carryback is secured by the property. If the buyer quits making the payments, the seller can foreclose which can lead to the seller taking back the property… and selling it again.