N950 didn't even get PR1.2 final, what makes you think it would get PR1.3?

The only way to get PR1.3 is to update your repositories to the same one that N9 PR1.3 uses, and install the mp-harmattan-pr package. But you have to avoid having sw-cert be flashed, because it would permanently brick your device.

I'm not sure what this seller is talking about, he may just be mistaken