Generally, you can get slightly more money when you sell used gear if you have the original box and all papers and accessories that came with the item. I have started to hang on to my boxes, but it's really more of a pain than anything else.
I packed mine away in a large box and put them in my storage area. But they do take up sdpace, so if you lived in an apartment, it might not be practical unless you actually stored your equipment back in the same boxes.
I have the boxes from my cameras and lenses, stored in the basement. I don't have as large a collection as others do, so it's not as large a problem. But I also have several Tonka, Nylint, and Structo metal trucks from my childhood (early-to-mid-60s) in their original boxes. My hoarding didn't start with cameras. From the rocking of the cradle to the rolling of the hearse, the going up was worth the coming down
I keep everything with,resale in mind, if its something I am going to look after - like a Nikon but not something thats going to look used like a kettel. Most boxes can be carefully folded for storage but the problem is the polystyrene preformed stuff that cones inside them. Just packed a D200 its lenses, grip, discs connectors etc all into original boxes,definately looks attractive for a prospective buyer.
What an interesting question. I have most of the boxes my Nikon gear arrived it -- less the stuff I picked up used. I guess they do enhance resale value, but I have not resold a lot. Right now the boxes reside in a large, covered plastic tub like you get at WalMart. That keeps them clean and dry. If I get too crowded I guess I will have to "take the cure" and relegate them to the dumpster some day. But in the meantime they remind me of the joy of opening the new gear. If you have room, why not keep them?
If I kept the boxes of everything I bought I would need a separate house just for the boxes. I keep it for a few months just in case I need to return it. But once i've determined there is no defect in the item I donate the boxes to the recycling bin.
The solution I'm using now is to consolidate the boxes as much as possible by nesting them within one another in a plastic tub. Had not thought of flattening the boxes as well. Resale value is a consideration I had not really considered, valid point though.
I do have my dad's old Contaflex with its original box, papers and bill of sale, circa 1953. It's quite impressive to see everything from that long ago still intact. I just wish the camera was in working order, it has shutter and aperture issues and I haven't been able to locate anyone who can repair the complicated internal mechanisms.
Right or wrong, my impression is that the equipment has been better cared for if the box and manuals are included. It also makes it easier to ship to a prospective buyer. I think many people like to get that new camera or lens feel when all of the packing materials are present.
A few years ago, I sold a 25 year old HP calculator for $120 on eBay. I think thats what I paid for it new. I had the original box and manual which the buyer valued.