Is it safe to purchase a Trezor wallet from someone else (that you might not trust)? Is there any way that something could go wrong if the wallet was tampered with somehow?
It's possible, although not an easy attack.
If you buy a possibly tampered trezor, you could just overwrite the firmware to make sure it's not been tampered with. But it is theoretically possible for the hardware to be changed such that it will accept the official firmware and STILL send the bitcoin to a different address than it displays to you.
But long story short, the few dollars you save buying a used trezor is very likely not worth it.