How do I know if an iPhone has been previously used?

Find My requires that your iPhone is logged into an iCloud account. This cannot happen until you set up the iPhone and sign in to iCloud. Therefore, if 'Find My' is already enabled when you first use the phone, it means that the phone has already been logged into someone's iCloud account. As a result, the phone would not be brand new.

I would be extremely suspicious of that seller, especially if they are offering a too good to be true price. It might be stolen or a cheap counterfeit knockoff. The iPhone 16 Pro Max is a brand new product being sold in stores. Why would someone buy one and then try to sell it right away? Ask yourself that question. How did the seller obtain that iPhone?

If the phone was used, the seller should have disclosed that – and should also have followed these steps before transferring the phone to you.

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

If the seller has passed off a used phone as a new one, you may want to read

The All Too Common SAD Reality of Buying a Used iPad/iPhone - Apple Community