I keep train and building boxes for re-use in case of selling a piece. Most buyers expect the original box as a condition of the deal - or a discount if there is no box. BTW, East Coast Train Parts is a good source for repro boxes. Although not "original," they are the next-best alternative.
I'm now 82 and have left instructions to my heirs about selling the collection (with the matching boxes) via an auction house.
I don't store boxes under my layout because I occasionally need to get under the layout when troubleshooting a wiring problem or adding a new accessory or lighted building. I gather boxes in black garbage bags, identify the contents of each bag with ID labels, then store the bags in the attic. (Many homes in central Arkansas don't have basements).
I have 30+ DEPT 56 buildings on my layout, and I saved every box and the sculpted styro. There's no better protection for these fragile porcelain buildings when the time comes to sell and ship one to a new owner. When buying a DEPT 56 building, I require the original box and protective styro -- so it will survive handling while in transit. No box, no deal.
I just bought a MTH Alco DL109 loco; thankfully, its original box, protective styro, and owner's manual was included. I hope UPS will treat the package with respect!
Many train boxes have a clear acetate "window" that shows the product. That acetate panel is often the first thing to come un-glued, but it can be re-glued in place. Next likely box problem - torn (or missing) end flaps. I once found an online vendor who specialized in repairing boxes - a cottage industry.
Mike M. LCCA 12394