Buy, borrow, trade

Some people like to be surrounded by books, and keep everything they've ever read. Others don't like the clutter, and keep them long enough to read them. Most people seem to be somewhere in the middle...

1. Do you buy the books you read, or do you borrow them from the library or from friends or from somewhere else?
2. Do you prefer new or used books?
3. If you buy them, do you keep them?
4. If you don't keep them, how long do you hold on to them before letting them go? What do you do with them?