You are probably referring to an ad that says if I cannot sell your home, I'll buy it?
You are hiring a Realtor to sell your home. So if you pre=agrree on a price that if it does not sell, they'll buy it, they are still conducting a transaction. So in most cases you will pay a commission. This is a business, not a charity. We would still have to do the work. BTW this does not occur very often.,