If you sell a set to somebody, let's say X, and X tries to sell that same set to Y, can you try to sell it to Y? I mean this is illegal, right?

how can u sell a set again if u already sold it?

because i have the set in my pc?

Yes, that's illegal. Once you've sold a set it's not yours anymore, therefore you can't sell it again. Common sense, really.

There have been cases that someone made 4 versions of the same head. As long as you tell the buyer that you are selling it multiple times BEFORE the buyer accepts, it is fine. If you haven't said this is some sort of disclaimer, you sold the right to use/sell that set.

Or maybe what he meant was that if you sold, for example, my set called "A" to a person named 1. Then 1 tries to sell set "A" to 2. That's how I get it.

Unless stated in a disclaimer that multiple versions of the set are to be sold you forfeit your right to the set upon selling it. The buyer then may edit it, sell it or wear it at his own discretion as long as he credits the original artist upon doing so.

So simply no you cannot sell a set multiple times. Closed.