Why?
Because of size?

I don't see how it make any sense. This guy won a Mi 9T so he's selling his Mi 8 but he's selling it at half the price of a new Mi 9T. And I can't understand why.

The Mi 8 has the better processor, I'd say at-least as good camera.

The Mi 9T has headphone jack (not very important), a bit larger screen and without the drop, a wide-angle camera (though not great picture quality), larger battery.
If it was the Mi 9 I'd understand because that's likely just better than Mi 8 straight up but the Mi 9T isn't the Mi 9. Then again to buy the Mi 9 I would had wanted it to be drop-less and have a MicroSD card slot. The 9SE cameras isn't at all on par with the Mi 9 ones. The 9SE is smallest though.