I like the concept behind what you are trying to when it comes to the safety of the purchaser but since you do not have any history in the forum it will be difficult to convince others to come onboard. Even if you had excellent reputation here it would be difficult for people to put their trust in the service you provide because of fear of tampering.

If you are simply providing a man in the middle service where you cannot gain access to the hardware wallets then it means the chances of them being tampered are still not reduced to zero because Amazon or other employees have the potential or possibility to tamper with the devices. In my opinion passing a promise of tamper-proof selling to potential customers (to convince them in order to buy) is something you will probably struggle with.