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Or, if it's a made-to-order kind of thing, have the shopping cart handle the "variables" selection (ie- I'd like it in red, size 3, with jingle-balls and purple glitter), and then handle the actual order/payment through a PayPal/ProPay type email invoice system, so the actual payment process also serves as a details-fo-the-order confirmation step.
The only automated option I could think of, if the thingies aren't customized/made-to-order, is have a shopping cart that includes an inventory tracking function... When you first enter the item into the shopping cart database, you also enter the number available. As each is sold, the cart increments the inventory database downward, and when an item reaches "0" the cart either removes the listing entirely, or automatically includes an "Out of Stock" notice on the item display.
But most carts I've seen with inventory tracking built-in are a bit on the expensive side.