yep! As I said it takes a lot of work, but at least lookup tables mean you only have to deal with new products and product id changes, not every product during every import.
Your look up table could be: -
Code:
MerchantID | MerchantProductID | MyCategoryID
---------------------------------------------
A | BCDEF | Z
B | KLMNOP | Z
Or even: -
Code:
MerchantID | MerchantProductID | MyProductID MyProductID | MyCategoryID
--------------------------------------------- --------------------------
A | BCDEF | 1234 1234 | Z
B | KLMNOP | 1234