First of all, 3d.sk pays women to pose nude so their skin textures can be used as 3D resources. I think it's understood by all that the textures may be used in nude or scantily clad 3D depictions.

Second of all, I can't imagine some girl seeing a nude V4 character and saying, "Hey, that's MY birthmark on her butt! I'm gonna sue!"

Because, see point 1. She was paid to pose nude so people could use her skin for just that purpose, and I'm sure she signed a release allowing it.

I don't buy directly from 3d.sk, somebody correct me if I'm wrong. But do the textures include full facial features that are recognizable when stretched over a 3D mesh? I figure these textures are modified six ways to Sunday by the merchant before releasing a character skin.

I mean, girls who do actual porn are so heavily made up that their own mothers wouldn't recognize them.  Can they be recognized in this context?

But even if they can, again, they deliberately posed for pay, knowing what the photos would be used for.