Prepayment without guarantees. The seller asks for money before meeting "to reserve" or offers to ship "via courier." After the transfer, they vanish. Never transfer money in advance.
Fake stores. A fly-by-night website with "low iPhone prices." They accept payment, never ship. Or ship a fake/dummy. Check: recently registered domain, no reviews, suspiciously low prices.
Fake photos. The listing shows a new iPhone, but the actual device is a beaten-up used one. Ask for a video showing the serial number and today's date on a piece of paper.
"Safe deal" phishing. The scammer sends a fake "safe deal" link that leads to a phishing site. Only use the deal feature from within the app itself, never from links in chat or messengers.
Wrong IMEI in photos. The seller shows someone else's IMEI on the box photo. You check it - all clear. At the meeting, the real IMEI is different - with locks. Always verify IMEI at the meeting: *#06# on the device must match the photo.