I bought some brake pads and rotors from an auto parts store, I never opened the boxes but will they buy them from me?

They dont buy parts this way normally. I'd take them back to the store where you got them. Even if they dont give cash back, you could probably get store credit.

Why not just return them? If it's not open, give it back, if it's an online store, they should provide a return label

Not for what you paid for them the best thing to do is sale them on line for a little less