Buyer should not be able to leave feedback for ANY transaction until after a reasonable amount of days so at least they buyer should have received their item.

In fact, there should be a mandatory option where a buyer MUST mark an item as received in order for the feedback option to become available. That would also help sellers know when their items have arrived, since some (a lot) of buyers don't bother letting a seller know, and personally, I find that infuriating.