It's not obviously true that the illegality of possession automatically implies the illegality of the offer to sell. Consider for example a case where the seller doesn't actually possess the item, but merely offers to sell it. In this case, the supreme court upheld a law making such offers illegal. Without the ruling, you might argue that the mere offer is protected speech.

The point of all this is really indirect though. I'm just saying that in cases where "speech" concerns illegal items, like nuclear weapons, child pornography, and drugs, sometimes the government is given more leeway in controlling speech related to items.