I've been cataloguing my books from my research, and found that I have bought and read around 900 books over 5 years. Thankfully there are some great places to buy used theological texts.
One I recently discovered is Pendelburys. Not only do they have a beautiful building that you can visit by prior arrangement (anyone fancy a trip to north london one day?), but they have 25,000 second hand and out of print theological books. They also ship all around the world.