Modern artificial rare or old rare?
I stopped trading games in many many years ago so there is probably some game I have in a box somewhere that is silly rare in the modern world, especially as my tastes tend to skew away from the sports games and towards longer form games, local multiplayer offerings or nice early examples of concepts. A few years ago I was laughing at the N64 suddenly being popular and it seemed the once worthless games can now be worth a pretty penny (seriously, what I might have struggled to even find someone to buy for a pittance now fetch considerable sums for things in worse condition than I have), even despite most being PAL.
Old rare... I have books going back to the late 1700s at this point (though most of them are 1870s onwards). I imagine they would be hard to replace, mainly as most of them are science and engineering related and thus nobody cared or current cares about them enough to preserve them (I get the impression most people treated them like you might treat a computer technician level windows 95 manual today) and thus is how my broke self can afford to amass the collection I have (charity and antiques shops love me).
By similar token I have a collection of tools and fixings from similar timeframes. I tend not to buy for rarity but what I find useful or just want to try out. To that end I would not be surprised to find some of the contents of my screws and fixing cases/storage shelves would have some restorer bite my hand off if offered, or some tool I occasionally smack with a hammer would instead go into someone's museum.