Craigslist is the first place that comes to mind. I've done this enough times to give you my perspective on it.
Browse through the phones and if you like one ask the seller for the IMEI of the phone. That's a unique identifier for each phone.
If it's a Verizon phone, call Verizon and ask if it's been reported as lost, stolen, or if the previous owner has an outstanding balance. I wouldn't bother buying it if it has a balance.
You might have to try this with a couple of reps but usually the tech support team can help.
If it's a non-Verizon iphone, it'll need to be unlocked before you can use it with Verizon. I always ask the seller if it's unlocked anyway. I don't like to be tied to anyone provider.
iPhone 6 and 7 are global phones meaning they're intended to be used worldwide but some providers like ATT lock them so they subsidize the cost of the phone with the service they charge.
Meet the person in person and verify the imei by doing this:
https://support.apple.com/en-us/HT204073If the IMEI is the same as the one they gave you, you're good to go.
Regarding security, always do a full reset on the phone before using it anyway as follows:
https://www.google.com/amp/www.techadvisor.co.uk/how-to/mobile-phone/how-reset-iphone-or-ipad-3296944/%3FampGood luck.