Hi,
a) Yes, it is already the case but it is not "visible" as it.
You can use the serial several times to put the product in the cart but only one product can be validate in the order.
- If the serial is used, you can't even add a product thanks to the code.
- If the serial is used when the order want to be created, the creation is cancelled.
- If there is not enough serials than the products in the order, the creation is cancelled too.
Just after the order creation, the serial is set as used.
Hope it will help you to understand how it works.
b) There is no plugin for that. I really don't think how implement such thing as a generic feature.
It is something very specific so it need specific development.
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.