With a "Owner Will Carry", the owner is willing to finance the home purchase for you, if you qualify, probably for above market interest rate.
The terms are between the owner and the buyer, but usually the owner is the one who dictates the terms.
For these type of deals, the owner may insist on a substantial down payment, say 20% or more. This is non-refundable if you decide not to purchase down the line.
The interest rate may also be considerably higher than market rate.
Most buyers who purchase using seller financing than seeking a conventional home loan financing have credit issues or financial hardship. Odds are when the balloon is due in 2-5 years they won't be in any better shape to get a mortgage.
In that event when the due date of the balloon comes and the buyer cannot refinance they are in default of their loan and the seller has the right to take the property back. The buyer loses everything.
In some cases, this type of financing does make sense.