Kids nowadays don't know this, but until the 1980's the idea of comic book stores was extremely rare. Up until then comics had always been sold on newsstands, drugstores, grocery stores, etc. It wasn't until this period, the 1970's, that a market emerged for comic book back-issues, and that's when comic book specialty shops emerged.
In their infancy, many collectors stocked up by contacting each other directly, in much the same way kids exchanged baseball cards. Today's entry is a prototypical back issue seller:
That's right, Sparkling Pajama-Man, oops, I meant Howard M. Rogofsky, has 250,000 comics in stock from all the popular publishers and even movie memorabilia. To find out what he sold, you can send 60 cents for a giant catalog.
Incidentally, these catalogs was among the starting points for comic book fan-zines as well. I was "lucky" enough to have been put on such fan-zines back in 1996 when I got a letter published in a Dark Horse Star Wars comic, probably just a few years before the internet had thrown fanzines into extinction.