If it's too good to be true, it probably is.
I have bought several used phones. I buy them from Craigs list. I always have the seller meet me at the carrier store and have it transferred and checked right there. So far I have never gotten a scammer.
This year my wife and I got a BOGO deal on Note 8’s. So half price not bad. We absolutely love the phones.
Gotta be careful buying used stuff.