Hi,
While configuring a price for your products, you can assign a joomla group for a product.
So you can put a price than only the users in the "doctor" group would be able to see.
The public won't see any price (or the "free" text). You will have to forbidden customer to buy free products so only the doctors will be able to buy it and everybody will be able to see the product page.
When approving a doctor, you will just have to assign him a new group.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.