typically there's a balloon payment due relatively quickly, meaning you make $x payments for 3 years (or whatever) and then the balance of $3500 (or whatever) is due in full.

If you do this, be aware that if your lender is made aware of it the brakes may be put on your loan. From the lender's point of view you all of a sudden have another monthly debt to pay that they didn't factor into your loan application. Whether you make the lender aware of a seller carry is up to you and the seller....