I agree with Mele. They have regulations they need to follow. But I also understand where you're coming from. I have 4 tattoos. One day I got freaked out about diseases from tattoos "my anxiety sparking" I went to the doctor got blood tests and all was good. Due to my anxiety I simply can't get another tattoo. But to help calm you down.... He has no reason to lie to you about using a used needle. That could cost him his job and have the shop shut down. I just don't see any reason to why they would lie. I'm sure getting a blood test done would fix your worries and you can move on.
No judgement here, but tattoos are a health risk, in general. I know they're "cool" for some people, and to each his own, but if you're really having anxiety after getting them, stop getting them. Go to your Primary Care Physician and have a Hepatitis screening done, too. Early detection is key and treatments are getting better all the time.
I'm sorry you're going through this. After I got my first tattoo, I was had anxiety that I had been exposed to AIDS or hepatitis, so I get where you're coming from.
The thing to keep in mind is that tattoo artists and shops are regulated by their state's health licensing group for safety standards. They can face serious trouble if they violate those standards. That doesn't mean to say that those rules are always followed - when you're dealing with people, that's going to happen in any situation.
If you're truly concerned you've been exposed to something, a trip to your doctor's office may be worth it for your peace of mind. And, if you get another tattoo from this artist, explain up front that you want to see the whole process beginning to end.
Good luck!