Used bookstores are like crack. There are too many books in a given trip that I feel I can't leave without, and it's especially bothersome when they're overpriced.
Nonetheless, used books are much better than new ones. They're what financial people call the "long option"; you buy a ton of them for cheap in the hopes that one of them is going to have a high impact. With new books, you can't be so surprised, the books are expensive and you come in set on some book that's been published in the last two years or canonized in the last ten. Maybe you get what you were looking for, but you have not much better of an idea of how satisfied you'll be than if you were to pick up a random book at a used bookstore.
I also find that used bookstores are more likely to have some more obscure things that are maybe a little bit dated but nonetheless more illuminating to a particular field of study. Today I got three books, some more well known than others:
Postmodernism: The Cultural Logic of Late Capitalism by Frederic Jameson
Empire of Signs by Roland Barthes
Essays on Non-Conceptual Content
The last one, by the way, is for research purposes. I also have a long option strategy when it comes to research on Bart. There are some books that I'll buy because I specifically think that they'll answer a question I have, but many things surprise me and give me new ideas on what to do. Of course, my reading list has gotten ever longer to the point that I'll refrain from posting it for fear of name-dropping.
I also very much enjoy buying used vinyl records, particularly aesthetically one-track house/techno/trance/trip-hop/etc LPs. I get them if they're a dollar or two and stack them up with some sound equipment I have yet to use again. Weird hobby, yes, but I dream of having some extremely lush jungle of random sounds that I can put together through all of these stripped down building blocks; making soundscapes through bricolage, to be precise.
Aesthetically, I'm a hoarder. I take many tiny fragments in the hope that the right set of them will make a whole that's far more valuable than everything I've collected.