Even the keys for sale may not be legal. They may have been bought originally with stolen credit card info or obtained another less than legal way.
I was lucky enough to grab this while it was still in the Steam store.
Also, holy cow you weren't joking. The keys are at least $150. They must be really rare nowadays. I remember looking at some 3 or 4 years back trying to help another person find a cheap key and they were only around 60 bucks.
Kinda sucks that people are missing out on it. It's a solid fighting game and a load of fun.

THQ/Activision lost Avatar's licences ( 12/2017 ), they can't sold it anymore