Is there an official statement link from the Ledger that says so?
The Ledger co-founder has posted on reddit many times that it is not recommended to buy Ledger products from third-party merchants that are not part of the official reseller network. They are not saying you shouldn't purchase from any third-parties, just not from unofficial resellers. Especially if the deal sounds to good to be true and costs less than on the official shop.
-snip-Thanks for the explanation.
It makes sense, especially if the price offered is too low than the official price.
I think, if you don't have technical expertise, especially in hardware wallets, those who have already purchased from a non-official reseller before completely dismantling the Nano S to check hardware integrity can use an application to verify the authenticity of the Nano S.
Application verificationWhen opening an application, a Non Genuine warning is displayed if it is not signed by Ledger. A modified User Interface (as found in
https://github.com/LedgerHQ/nanos-ui) will also display a warning message on boot.