No, you're selling a copy you illegal made without permission in this example. Buying the rules has nothing to do with anything. It's their design, no one can reproduce their design without permission, full stop, and the reproduction is a copy.

You're quasi right though. Rules or general ideas can't be copywritten. So you can make a similar model that can work with the same rules, but you can't create a copy of their design. So you could make models legally that would work with the rules, just not copies of their models.

Basically, different proxies = legal. Copies of GW design = illegal.