>> Is there any point from a resale perspective?
I remember when I first opened my BrickLink store, and I saw it was an option, I couldn’t imagine it was worth the hassle of keeping the boxes, and storing them (I lived in a small apartment at the time), and I couldn’t imagine anybody would really want them and be willing to pay to ship them. But a friend of mine has a BL store also and he told me he’s made money selling the boxes. So if they’re UCS sets, it’s quite possible that there’ll be an interest. I’d look up the boxes on BrickLink and see what they’re selling for and see if any have actually sold in the last 6 months and at what prices, that should probably give you a good idea if it’s worth it for you. I would think that flattening them would reduce their value, but I’m not sure about that.
I have some of my old classic boxes – Galaxy Explorer, yellow castle, etc – but I cut them apart and only saved the front and the back, so that they’re easy to store.