I used to collect many things. Now I don't as much. After you get a few thousand book it starts to get cumbersome. Always wanted to collect fossils but soon realized that there was a sizable illegal fossil market that pillaged a lot. Collecting them would feed this problem. Then I wanted to collect cone snail shells. But it was pretty darned obvious that the harvesting of cone snails is unsustainable and that collecting them in any way would contribute to this practice. Nowadays I am more content just looking, rather than owning. So I go to museums and auctions.
I think ebay has something to do with it. When almost anything is freely available, why bother collecting it? I think this has made me hone my collecting to things that still present a challenge.
One exception for me would be primary sources. I will always buy a primary source if I can afford to. Diaries, photographic negatives, even recordings.
A few nights ago in Random jibber jabber, somebody (single malt?) brought up bomber nose art. I have a few rolls from WWII in India or China of a whole bomber crews' individual photos of each of them burying their faces in the crotch of their pin-up girl in feigned cunnilingus. They're funny. I'll dig 'em up. I still collect this type of stuff. When I get old I will compile something cool out of them