Thanks for the chance to help. I am an attorney with over 12 years experience. Hopefully I can help you with your legal question.
The sign alone is not going to protect you...since if law enforcement can prove you are in fact selling the items as "illegal drug paraphernalia" as opposed to "legal drug paraphernalia" (tobacco is currently legal, marijuana not), then they could prosecute. Possession
and sale drug paraphernalia is illegal under both state and federal laws.
Now...in practice few "head shop" owners are ever prosecuted. There are many reasons I expect, including that there is a requirement of proof. A pipe that can be used to smoke marijuana can be used to smoke tobacco as well. So they would have to prove that the items were intended for use for consumption of illegal drugs. One great feature of our criminal justice system is the requirement of proof beyond a reasonable doubt. Your selling an item that can be used for tobacco...not your control what the consumer does with it.
The other reason why head shop owners are typically not prosecuted is that they generate money for the city/state (assuming you are paying taxes).
Frankly, I have never seen prosecution of a head shop owner.
But my point is that the sign, in and of itself is not a shield. You could certainly point to it in the unlikely event you were prosecuted. But the fact is, prosecution for head shop owners is the exception.