iPhone 12 mini trade-in without original bill

I purchased iPhone 12 mini from a local dealer in my town. It was a second hand phone cuz it was my first iPhone. Now I want to upgrade but I don’t have the original bill but I do have the original box. Will I have any trouble during trade-in or will I need an original bill to trade in?

and is there a way to get original bill as I’m not in touch with the first owner neither is the shop keeper?

[Re-Titled by Moderator]