Judy,
It’s totally up to you and the seller. It should be spelled out in you agreement(s) with them. I pay my part of closing at the colsing. If there is a underlying note, I usually have the seller pay that at the appropriate time (satisfaction and filing fees).
Has anybody out there ever purchased a property with a wrap around mortgage? If so, was there closing cost involved at first or does that come later after you pay the first underlying mortgage?