In order for you to buy products from your customers, either through the website or via your POS, the products in question must have an optimal quantity set (please see this link for further information).
You must make sure your optimal quantity is greater than the qty of the product that you have in stock.
From the image above, each condition has an Opt Qty of 30, but their current Qty is 0.
Therefore, for each product, the admin is able to purchase 30 of each variant.
For further information, please also see this article or speak to a member of the support team.