Allowing for choice of quantity exported to ebay is good. Quantity on ebay, in CS-Cart and total in both channels should be available. It would also be good if inventory could be shifted from channel to channel if desired. All simple arithmetic on one extra variable. If no stock in CS-Cart channel but some in eBay, then attempt to purchase in CS-Cart could result in temporarily unavailable message with optional link link to ebay.
At the moment we plan the following behavior:
Each time a customer places an order in your CS-Cart store the Ebay add-on will Revise item at Ebay and decrease it amount.
Each time you import orders from Ebay the quantity of bought products will be decreased.
Quantity split feature is not considered right now.