I save the boxes, if I have space, of things that I might sell in the future.
Once when I was in high school, our house was burglarized, and my fairly new Canon A-1 system was stolen along with some other cameras. I had the boxes, so I had the serial numbers, and that was helpful in catching the thieves and claiming the insurance. The cameras were long gone by the time the guys were caught, but they still had some other things that were returned to us. The insurance adjuster was sceptical about the idea of a kid my age having such a nice camera system, but the boxes and working darkroom were solid evidence.
Now I keep an inventory list with all my serial numbers, and to keep track of what I've bought and sold and forgotten that I've sold when I've gone looking for it after the fact.