Item Sales History
I think it would be a good idea to put an items sales history in the product description page. That way you can easily track what invoices an item was sold on. This would help in finding quantity errors as well as general inquires as to who parts were sold to.
A report would also be very nice.
maintain stock doesn't work for serialized items. still looking for a way to run a report to see the serial numbers of all certain sim cards sold.
Quantity Details on the maintain stock products may help you.