I have a large collection of electronics books which are often expensive and electronics is only a hobby for me so it's hard to justify the cost. Sometimes you have to buy second hand books, many of the definitive Z80 books are out of print.
I started buying second hand books as a child. I wanted books on philosophy and this was the only way I could own works about Descarte, Locke et al without begging my parents for tens of pounds (my income way about £1 a week back then). I've even come to like seeing other peoples notes in the margins. I often wonder "What are they thinking?" when they highlight different sections of the text.
We always try to buy furniture 2nd hand- for the price of a chipboard IKEA wardrobe you can normally get a big, solid wood, Victorian one which is quite likely to last a lot longer!
I myself own more second hand books than new ones.
Used books are very popular among students, though, but that is once again something different.