Use the Apple Trade-in estimation option that is available while buying a new MacBook online. I have a similar model and Apple offered $580 in trade for new M1 MacBook Pro. You might get more selling it yourself but it’s a good starting point. One nice thing is you can option for instant trade-in credit if you order a standard configuration and pick it up from an Apple Store.
Costco web site offers a similar instant trade-in estimator (through a 2nd party) but no cash only a Costco gift card, and you have ship it to them and wait and wait. I used this for my old iPad and the process took a month.