Is it safe to use a second-hand Trezor (like from ebay)?
If you update the firmware you are probably safe, but why take the chance? The Trezor itself (or even the included USB cable https://www.wired.com/2014/07/usb-security/) could have been corrupted/replaced by a sophisticated actor.

If someone gave me a used Trezor for free, I would throw it in the garbage. No discount would be sufficient enough for me to justify buying a used Trezor:

https://www.reddit.com/r/Bitcoin/comments/2dbjd9/trezor_tamper_proof_seal_doesnt_help_much_against/