[gb2world]

It seems that setting an artificially large number for inventory levels will be a little messy if you ever what to track true inventory with your X-Cart database.

Instead, you could use a product extra field and set that to some value to indicate a product is available for pick up.

Another option is to use BCS Preorder/Backorder mod. I think you should be able to easily modify the backorder functionality to be the "available for pickup" functionality, but you could check with BCS.