I'm about 50/50 with buying new and buying/swapping 2nd hand.
There are certain series by aithors that I collect and keep, therefore I buy (or get them given to me as gifts) new, preferably in hardback. I often get book vouchers, so I spend those in places like Waterstones or on Amazon. I also buy cheao new books from discount stores like The Works, especially for classics.
For buying on a budget, I tend to go for Green Metropolis - if I'm getting hold of a book for a reading circle and it's an untried (for me) author, I'd rather pay a cheaper price and 2nd hand is fine (although I still try to get them as new-looking as possible - LOL!). I also swap some books through Book Mooch. I used to use Read it, Swap it, but I got sick of having to choose books from lists where nothing really grabbed me as unmissable when someone wanted a book from my list - the points system of Book Mooch appealed to me more.
I have been known to use the library too, but a it entails a trip into town if I want a decent choice (the smaller branches usually have pretty much nothing that grabs my attention) and another trip back into town to return them (usually on horribly busy Saturdays), I only really go when I'm after a specific book on a temporary basis and I think they'll have it.
I used to use the library a lot more when I lived literally around the corner from a very decent branch library a few years back. I went in roughly every other day and the assistants knew my tastes so well that they would keep books back for me that they thought would interest me, as they knew I'd be in within another day!
I also donate books to charity on occasion...
The thought of ever just throwing a book out in the trash makes me feel physically sick!
I used to keep ALL my books, but I ended up with hundreds and hundreds of books in storage, never being read again, whereas some got read in rotation time and time again. I decided I needed more room for books I hadn't read and for new books I wanted to read, and that keeping hold of old books I knew I'd never read again, I had to find new and loving homes for them all! That was when I started selling and swapping them. I've sold nearly 100 books on Green Metropolis (and others via other means), swapped about the same amount and donated piles of them to charity shops, just to clear some space.