Sébastien Page:
Even though they might not be able to activate the lost or stolen device, they might still try to put it up for sale, and if you don’t know what to look for, you might very well be buying an iPhone that has been locked and cannot be activated.
Good advice.
best advice is if you are going to buy a used iPhone only do it if you can collect and pay for the phone in person. Don’t hand over the cash until you personally power it on and attempt to activate it (you’ll need a sim card, carrier specific if it’s locked, but it doesn’t have to be activate). The activation lock screen comes up right after you pick a wifi net. Places like starbucks can be iffy for getting on wifi for activation (cause the whole ‘i agree to the rules’ screen doesn’t pop up) so you might want to bring a laptop and go that route. if you can’t get to the home screen, no sale.