Seller Financing question

[Pawley Golfer]

I am confused about seller financing. I like the theory about it in principle but it would seem to me that it only makes sense to do it with someone that owns their property outright? Anyone with a mortgage would be hit with a due on sale clause and or could create real potential problems for the buyer. I would imagine entering into any deal you want the deed transferred immediately with the lien on it. Can somebody shed some light on the pros and cons of doing this and if I am way off base in this thinking? I'm interested in doing this because the fees with the bank are just ridiculous as well as the qualification process. Also, if I were to enter into a seller financing deal, what is a good basis for the type of terms to look for in regards to interest rate, number of years of the loan, down payment Etc. T.I.A