There are a couple of steps you will need to take to implement something like this. You will need to add a recordset to get at the inventory for a particular item, then you will need to have an update statement to update the inventory, you will just subtract the quantity.
This code will need to make use of eCart variables and be within an eCart repeat region. This is a custom implementation and it will be up to you to get it worked out. I can help you with some pointers if you have any problems, just let us know.