Always buy/trade a seller not the item. Ask for a VOICE phone call to a verified carrier number. Avoid if they're using scam now aka text now or some app to mask their true carrier number. Ask for references if you can't see any verifiable trading history. If it FEELS fishy, RUN don't walk away.
On another site (QRZ) I ONLY do communication through site PMs, this knocks of 99 percent of scammers/flim-flammers but at the end of the day, one has to do all due diligence to protect themselves.
A FaceTime/Video call with the products in hand being demonstrated is also a great tool to not only ensure no one is disappointed, but no scammers ever want to show their face so if they decline, RUN far away.