i have written seller carry back purchases before.
Basically, if the owner has not yet paid the mortgage then what you are doing is actually called a "wrap" mortgage.
What you need to do when your Realtor writes your offer is work in conjunction with a title company that can close and service the transaction.
You are basically paying your payment to the title company. Then the title company insures that that payment goes to the sellers lender.
Any remaining balance is usually forwarded to the seller or can remain in escrow depending on the terms of the agreement.
Most involve a considerable down payment amount and usually a purchaser on a wrap sets a time and date as to when the seller will be paid out the remainder of the purchase amount by you refinancing the home, which you can do because the wrap will put you on title.
Should you need an experienced agent to help your with your purchase, I would be happy to help.
All The Best,
Home Smart Real Estate
480-220-7491 - firstname.lastname@example.org