How would you know if that seed was pre-made by someone else, say the person that sold it to me on Amazon? Would it have been pre-written on the seed paper I save somewhere safe? I've heard they would give you a pre-made pin?
Two ways a hardware wallet might be compromised by an intermediary:
1. The seller preloaded a seed and PIN. They'd have to tell you the PIN for you to be able to use the device. To look legitimate, they'd also have to give you a printout of a seed. The given seed might not be the real seed - they just have to make you think you can recover the devise. If the devise made you choose a PIN and write down a seed, then you are safe from this type of tampering.
2. The seller loaded malicious firmware. You may think the device is working normally, but it is really showing you a preselected seed. Ledger has some mechanism to check the firmware when you go online. Trezor comes with no firmware loaded, forcing you to (hopefully) download an untampered version.