Updating Product Table

I am using PayPal standard and most of the products on sale will be unique items so I want to put them to Reserved status on the website as soon as a client puts the item in their cart. I want to control the product change from on sale to reserved by the stock level changing to '0' that I can simulate OK. However, it is updating the stock level that is causing problems as I cannot get all the items in the Cart to update the appropriate product table stock level, it only changes the 1st item in the cart product list the rest of the items are ignored. It seems I can only use the Webassist update a single record that is potentially the problem hence only the first record being updated and the update multiple records server behaviour does not recognise the form table in the cart as a repeated region. Is there a way round this?

Thanks in anticipation of your help.

Keith