You would check out the buyer's credit and you would collect first month's rent plus security deposit.

If you advertise "We Lend You The Down Payment" then you would lend all of it.

You would charge 12% on this non existent money, collect payments for a year or 2 then sell them the house as agreed.

If they did not go through with the purchase you would keep all the payments for nothing.

Do I really have to explain why? OK, because it is fun to make money . You have no risk and you collect payments and interest on money you make up out of thin air.