Any way to prove it has not bee in use?

[Adfernz]

My Blaze (new) stopped working, Fitbit offered me a 25% discount on a new Fitbit, what a joke.  Thanks, but no thanks.

[ChrisAtkins]

My Blaze also stopped working correctly recently, after almost a year of
working well. Customer support was excellent and it transpired my Blaze was
just within warranty and a new one was dispatched immediately. It arrived
promptly and works just fine. I'm surprised that if your Blaze was
genuinely new and unused that the warranty didn't cover it and that your
treatment was so different to my experience.

The only aspect of my replacement that surprised me was that the old one
didn't have to be returned for diagnosis of the problem, which I would have
expected from a company like Fitbit and would have been quite happy to do.

[Rich_Laue]

I could see if Fitbit was still making this model, then yes they might want to have the tracker back.