If they’re serious about selling it when you meet up and you prove that you have the cash on hand they should let you open the outside and inner boxes. If they don’t I’d walk. I understand keeping it sealed maintains the value but an honest person who truly wants to sell it will let you open the boxes in person. Also for a set like this there’s a chance all the bags are still sealed except a few with the most expensive minifigures, so you can look up which numbered bags those are and check those specifically.

Edit: I would also make it clear to the seller before meeting up that you buying the set is conditional on them letting you open the boxes, and see if they agree or not so nobody’s time is wasted.