To get your list, either on the screen or as hardcopy, take a look at how the invoices starter solution prints an invoice. It pulls up the invoice's set of line items on a list view layout based on the line items (called invoice data in Filemaker 12) table.
For some ideas on automating bar code scanning: Need help writing script to integrate barcode scanning
You can scan your barcode into a global field. For examples of how to use a global field in a scripted find see the examples in this thread: Scripted Find Examples
Or you can set up a relationship based on that global field that matches barcode values to a matching field in your products serial table and you can change the value of that instock field without performing a find.
thanks for your reply, what would you suggest if id like to make this a little more complicated. lets say in the current estimate there is only 1 line item with a qty of 10.
what i would like is when the staff go to the scan/shipping screen the initial qty of this line item is 0/10 and when they scan a barcode it adds to the count eg 1/10, 2/10, 3/10 and when it reaches the 10/10 the line turns green to indicate that the line item has been fullfilled.
the latter part i think should be easily achieved by a bit of conditional formatiing but its the count part that confuses me.
See my thread on bar code scanning scripts. You'll need a script that processes each scanned barcode. This code can add one to your QtyShipped field each time it is performed until it matches the Qty field of the estimate.