How do I buy used?

I want to buy a neverfull mm. I have an appointment to go to the store next week to make sure it’s what I want. However, with how “basic” the bag is and the price increases, I was thinking maybe it would be wiser to buy one used. However, I am a novice in luxury brands and don’t want to be duped. So, before I start looking, how do I buy used as smart as possible? Thank you for all your advice.