Medical despenary laws in WA state

I have just been made a care taker for a medical user. I've been reading over the laws and it states that it is still illegal to buy/sell marijuana for medical use. But there are list of people with business licenses that you can buy from all over the sate. How is this possible? I know a lot of medical users in my town and would love to become a despenary myself if it won't catch me a felony.