It's both a shame that used bookstores have been dying off, and a blessing. Mainly because every shlock genre book that fails on Kindle is a print run that spared a copse of trees.

My personal comparison:

Online Bookstore: Hard to find, niche, targeted purchases.
Ie. I need a reference book on 12th century Persian woodwork. I also heard about a fiction novel with a giant spider as the protagonist through a forum and its only print on demand, damn it, not that I can fault them because being your own publisher sucks. Oooh, what's this, now...? Erotic Klingon Verses... Why? Eh, why not?

First Run Bookstore: Purposeful, general, targeted purchases.
Ie. I want a dessert ideas cookbook.

Used Bookstore: Book Pathfinding and browsing.
Ie. If you find one copy of a well-read book, it has promise; if you find 20 copies of a rarely read book, you've found a stinker. Plus, frugality or bulk purchasing perks...

Library: Entertainment discovery and research at a leisurely pace.
Ie. I need a dictionary. Check that, all of the dictionaries. And topo maps. And microfiche, probably, because that local history research I want to do predates the Internet. Wait, they got a laser cutter? Damn, there goes my afternoon...