[Russell Sobie]

That's pretty much copyright infringement, if the skin was completely original work from the original author. For the most part, liveries on pretend race cars are typically full of BS like company logos and clipart gathered from google image search, so you'd likely have a hard time raising any sort of legal action or anything super serious like that.

Basically I'd just say that's "bad form" and would discourage the practice. At least without the original creator's permission.

When I was making skins for others just about every week I would tell them they could do whatever they wanted with it after I gave it to them. But I'd be pretty miffed if someone ganked one of my designs and made it look bad (i.e. different in any way than my original creation).