Ah yes, the pain of book-buying.

Avoid the bookstore at all costs. They give you maybe a 1/4-1/2 of what you paid for it and turn around and sell it for 3/4 of the "new" price.
The engineering honors fraternity at my university held a semesterly "book swap" for all subjects. Check to see if there's something like that?
I used to go to the bookstore, write down all the ISBNs, and then start hunting online.

The library may well have copies of some books, but I wouldn't rely on it.

Check the "used" section of Amazon. Ebay?

I'm not much use, I'm afraid. I had to buy several of my books junior and senior year new because the instructor required the current edition because "There's stuff that wasn't in the old editions." Surely basic biochemistry hasn't changed?