You should be able to do this with a regular update query like the one that you have. You should do the math for the new inventory before you have the query, then use the new value for the inventory update.
So to start you would need to query for the inventory that is present filtering the rs on the product id. You would then store this inventory value in a variable and subtract the quantity purchased from this value. You would then store the difference in a new variable and use this variable in your inventory update.
To make this work with all of the items you would need to have a custom loop that would make this recordset and update query execute once for each item in the cart.
