Counterfeit devices are widespread.
Product serial numbers are routinely be copied, too.
Buy only from Apple or an authorized reseller, from a reputable seller with an established history and a good return policy, or buy from elsewhere only at an immense discount appropriate for the large risk you are taking as a buyer.
Online auction and marketplace sales are routinely counterfeit, and the sites (absent some guarantee offered by the site) will seldom assist in resolving purchase problems past requiring the fraudulent seller to get themselves yet another vendor account.
For some background on counterfeits, Adam Savage of Tested:
https://www.youtube.com/watch?v=Db99cXMD780
This article is about iPhone and iPad, but applies here, too:
The All Too Common SAD Reality of Buying … - Apple Community