How do I change a phone bought from an individual seller over to my data plan?

I'm looking to buy an iphone 6 or 7 on Facebook Marketplace (I currently have an android and hate it) and was just wondering if I could just go to the verizon store after purchasing it from someone and ask them to switch the information for me? Would they do that? I'm looking to buy an iphone compatible with Verizon because that's the carrier I use

Added (1). Should I just buy an unlocked phone? I'm also looking on Amazon

I wouldn't get ot off facebook market place, they have a bad reputation for sending out bad or fake goods, no goods at all and are used by scammers.

If you buy it from an individual: You make sure that it is not linked to a network, and you watch them as they unlink it from their account. You then IN THERE PRESENCE put in your SIM card and check that it will link to your network. You can then buy it! If that does not work, or they can't do that, then you don't buy it! If that works all you will then need to do is go through the set up on a computer to link it to your Apple accounts, you do not need to visit a Verizon shop.