You haven’t given enough information about your display issue at all.
Like:
You mentioned this iPhone was ‘used’, where did you purchase it from? If it was from a refurbished third party store, I’m wondering if they replaced the display with an authentic Apple OEM display?
Do you notice the ripples when you interact by physically touching the display or simply looking at a display with a white background?
Do you hear any type of popping or clicking noise when you Apply pressure to the display?