Now that so many old series are available as ebooks I've been buying and reading whole series. Most of them are a combination of books I read a long time ago and books that I've never read.
I think I probably started moving from Nancy Drew and the Hardy Boys to "adult" mysteries when I was about ten (which would have been in 1954). Agatha Christie, Rex Stout, Patricia Wentworth, Michael Innes, etc. were still writing then and I read what was available in my small-town library.
Sometime in the sixties, after I left school and moved to a larger city, I started buying books, mostly current paperbacks, eventually adding in hardcovers and searching out mystery bookstores (or general bookstores with large mystery sections) in any city I visited. Used-books (and browsing in most of the stores that sell them) have generally been off-limits to me bedause of allergies.
Because of space limitations and moving costs I did a lot of donating and some re-buying. I never did end up with any whole series at once.
Having most of my favorite old series wholly available as ebooks is amazing! I suppose that most people in generations later than mine take this for granted and only buy or borrow books as they're ready to read them. I feel compelled to buy and hoard.