Ford made me sign a contract similar to this when I bought a Bronco sport. I was annoyed the dealer sprung it on me at the very end when I was getting the keys, but didn’t plan to resell in the first year anyways.
Is that even legal, to ask you to sign that after you paid for it? Or was it before?
You can be asked all kinds of things. It's a little trick law enforcement tends to use: ask for things that the person being asked doesn't have any obligation to do.
As long as no consideration has exchanged hands or signatures signed, then yes. Contracts can be modified up to the second they are signed as long as both parties agree to it. Obviously, no one could negotiating away contract like this, but you always have the option to walk away.