Some IKEA items have a metal insert + machine screws and slots for the back rather than nailing, so those can be easily disassembled and reassembled without issue. IMO, if IKEA was serious about sustainability they would make them all like that. I think they are more looking for PR and know that the same reasons that keep people from moving the items will keep them from returning them as well.

I do have a couple of short shelves that I got used and moved 3-4 times that look to be screwed into the material (my memory isn't that great :/) and I'm guessing at one point had a nailed on back. I'm fairly sure it just worked for me to disassemble and reassemble without change and they are somehow stable without any cross bracing. The previous owner might have changed the screws.

I also have a pair of IKEA chairs that were quite unstable when I got them used and the issue turned out to be that the order of tightening screws made a huge difference. Luckily it was new enough that it was easy to find the manual on IKEA's site or I would never have figured it out. I think a number of items have tricks like that where you need to exactly follow the instructions or it doesn't work.