Being that I work for NYC transit I collect books and maps that collectors may want 20 years or so from now. I also collect transit related books, magazines, badges, tools. I have a large collection I hope will give me a little money in the future.
Recently I found some old destination signs that were just collecting dust in the corner of the office, the line has been elimated by NYC transit so collectors will be willing to buy these from me. I found severla years ago bus maps that were being thrown out in 1996. The maps were 6 years old at that time making them 14 years old today.
I also purchase old maps or trade with other collectors.