no offense but if you give me a plugin where I put a quantity ... if I put 1 and is bought and appears sold the plugin must block a new payment. Alternatively, the plugin contains a bug. If you say no ... it does not make any sense to set a quantity.
because if I'm offline and 50 people buy, I find myself having to repay 49 people, putting back in my pocket the orders of those who provide the payment service