Do you keep them?
Some of them. I have over 150 fountain pens. I have three medium-sized shipping cartons packed full of retail boxes in which some of those pens came; they're all in the space under the bed in the spare bedroom. I was just repacking them the other day and cleaning up that room, ahead of a friend arriving to stay for the weekend. But then I've also thrown out quite a few of the boxes, both during that exercise and previously.
If so why?
I'm a hoarder.
And, if I need any 'justification', it is the imagined potential price differential between selling a used pen with the original box and stuff and selling it without, not that I care either about selling any of my pens at this stage or about the money (as in what I stand to 'gain' by achieving a higher price; not recouping my original purchase price on something I bought new but has since used is not seen as a 'loss' to me, but just the price of my consumption or derived utility while I had the item).
Edited by A Smug Dill, 15 March 2019 - 23:18.