These models are available online for a very long time, long before he even uploaded those here. As far as I know, as game company employee (if he is one wich i doubt) is not allowed to sell asset made for game because those are property of right holder in this case Activision. What i think he has done here is took those ripped models and modify them so he can sell that here.
As you can see in the link below here is the original gun from COD BO3, just to proove that those models are available online.
https://ibb.co/cD6WLYR
If I were you i would pass this, because its to risky in every single way.