I just bought a used one and when I rack the slide and then pull the trigger the slide moves enough to make the dot in the reticle jump.
When I start pulling the trigger the slide moves down closer the frame. When the striker drops, the slide moves up away from the frame to its original position.
How do I stop this from happening?

Completely normal for a P320 during dry fire.

Has to do with how the striker leg interfaces with the sear. Sig Mechanic explains it much better than I can:

https://youtu.be/itGqqq8IcL8?si=dn3Lu-HjYQcDa84v