I’m of those who upgrades every year and tries to sell my old phone. I think $750 might be a little on the low side, but it’s in the ballpark for ‘normal’.
I will say that right after the new phones are released the value for the previous model tanks because of all the people who upgrade and want to sell their old phone. There’s a lot more on the market and that makes the value drop. Unfortunately, right now is probably about the worst time to be selling an 11 pro max, since the 12 pro max just came out.