How do I sell my iPhone 12?

You are free to do whatever you want with it, just make sure you prepare it properly before selling or trading it in.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If you are interested in the Trade-in options offered by Apple, you can look here. I would recommend the In-Store option where you can receive a Gift Card. You won't get as much money this way as selling it yourself.

Apple Trade In - Apple

If you own it, you can do whatever you want with it.

Follow the instructions here to prepare it for sale: What to do before you sell, give away, or trade in your iPhone or iPad