In the Business edition of HikaShop, you can restrict the prices of products to groups of users. That way, you could set it up to have the prices only for the group of users "buyer".
When a user from another group would saw the products, no prices would be found and the text "Free" would be displayed instead of the add to cart button. Then, you could override that text to something else like "It can only be bought by buyers" via the language edition interface in the languages tab of the configuration.
