Yeah, I don’t think you have any legal recourse with T-Mobile. You could only take the guy who sold you the phone to small claims court, which would probably take so much time that the wages you lost would be more than $800.
And then, even if the judge found for you in court, the guy would still stiff you.
This is why, if you have the choice, you should buy things brand new. This isn’t the 80s anymore. Everything you buy has some kind of dependency on a service which only the service provider can give you.
You may save money on one of these purchases, getting it second hand. You may save on a few of those. But you’re always taking the risk that you get screwed.
And it really sucks that basically all 3 providers of cellular service (if there are even that many now) are colluding to keep prices higher than equilibrium. There’s no way they aren’t. It’s just too lucrative to forego, and everyone knows the US government won’t enforce antitrust laws (except for Lena Khan whose days are numbered).
So it’s always best to buy this stuff, especially electronics, through an official retailer. And even they can fuck you. It’s just less likely than some dude on e-bay.