How can this even be a real problem? Put the item on the MB for your gazillion gil and have the buyer go get it off the MB. Be in the right city and there is not even any tax issues. It is not like anyone would try to steal the item off the MB for gazillion gil, and if they do the seller doesn't lose, still go the gazillions. The buyer might be bummed, but they didn't lose their gil. Done and done.