There were items released before I collected them, for example some Master Replicas items. I wouldn't buy now a MR prop that was still factory sealed, because MR is long gone. Get it now?
i believe most of use are talking about "when you have a choice would you get it MISB or used". your case of pursuing older figure/item that has been long discontinued/sold out is a different story, beggars can't be choosers. when you're late on the bus you have to take whatever is available, MISB or used, depends on your budget & availability & how long you are willing to wait.
but i don't understand the part where "I wouldn't buy now a MR prop that was still factory sealed, because MR is long gone", because even if an item is "long gone", there's bound to be MISB ones available, unless it's uber rare limited quantity or ancient old, or you're saying even if a long gone item that is MISB is still available you would rather get a used one??? you do know for very old items, it's difficult to have it in tip top condition even if it's displayed in a dust-free temperature controlled environment. So if it's an really old item i would favor MISB over used, even if the price of the MISB is 20% more, for really old items.
and if you are paranoid with missing parts or damaged item you can always ask if seller is willing to check & verify the item. It's not difficult to open up MISB and put everything back MISB, those tapes can be easily removed without damaging and sealed back. If it uses those normal tapes you can just use a new tape.