Sold out items can be purchased if they're already in cart

I'm working with one-of-a-kind items, so each product has an inventory of 1.
I tested having shoppers add item to cart.
One then checked out and item is now "Sold Out".
The second received no warning when they viewed their cart, in fact made it all the way to Paypal.
Is there a way to warn users the item they are attempting to purchase has been sold and then not charge them?

There is much to love about ecommerce, and I appreciate all the work that's gone into it, but it definitely looks like it's meant for handling larger inventories of products that don't suddenly dry up, and my industry deals with single items and small runs.