Can the Product be purchased once per Order or once per Customer?
For once per Order, set the Maximum to 1 ...
For once per Customer, you would need to customize the code to check the orders_products table to see if the products_id has been sold to the customer, and if so, turn off the Add to Cart/Buy Now button ...
This could be done in the button function to change its display based on the customer ...
Do customers have to login to your site to add to cart?
If not, then you need to also have a test to remove the product from the cart if it gets added before the customer is logged in and then test the cart for the product to see if it has already been purchased ...