A: For each item in the database you can set "in stock" quantity. Upon the purchase of the X quantity of the particular item through the PayPal, X number will be subtracted from the in stock number. Once new in stock number hits zero, item goes out of sale and is no longer available for the purchase until you will replenish in stock number from the admin area (via edit item function). Additionally you can not add to cart number of items that is > in stock number.