It not totally clear to me what you want to achieve.
Anyhow, you can have two portals in the same layout: one that references TableA and the other that references TableB.
What the user select a product in PortalA, you have a script that copies all the field from TableA to TableB and the record will be present in PortalB.
BTW, I do not expect that the user enters the stock quantity, normally this is a system data and the user can order up to this quantity.
There are many questions here...
Why are you deleting Products no longer being sold? Shouldn't you just be changing their status? Popups should be showing Product values for only Active records and not Deleted (not physically) lines.
An Order is just an Invoice by a different Status. A Quote is also an Order and Invoice with a different status. This status can be used to filter (by relationships) the records being included in each context.
Your TABLE data should really be in a single table. Data can be looked up or auto-enter calculations and remain intact. While you might choose a product from a popup, the data entered as a result can be locked in place. I really don't understand the value of B except as a Backup / redundancy.
If you really want to do this... I would do it with an export to a temporary file followed by an import.
The error you are getting when you attempt to import directly sounds like you are trying to import into a table using without the "add records" radio selected so you are trying to import eg 10 records from A into Table B to replace 20 records.
The people who have asked to set this up do not want it interfaced with their stock management or accounting software. They count their stock at the end of each day, and how many they need for the next day. This piece of paper get faxed to the guy who goes to the market and does the buying (filemaker go will be used to replace the piece of paper). Hence there is no way I could get the in stock numbers automatically
so what you really need is a master shopping list that dwindles down to the items with a needed quantity. Right?