The example of a "one-of-a-kind" autographed product is perfect.... I am thinking about what happens when I am buying tickets for a sporting event online -- specifically, I will use stub-hub or the SF Giants "Double Play" ecommerce site to search for seats/tickets, since these are one-of-a-kind, the assumption is made (by the system) that when I select a seat(s) that I am going to buy them, however, I am given a warning "you must purchase these seats in the next 10 minutes...."

Where I am going with this -- a possible solution would be to have a class that is "unique item" and has an admin option to "remove from cart after nn minutes" ...
OR
Have a function that 'checks inventory levels' upon 'add to cart', if inventory=1, then display a notification : "This is the last one, buy it in the next 30 minutes or the [product_name] will be returned to the store inventory"

Meanwhile the product does get 'locked' so someone who clicked "add to cart" one millisecond after the other buyer added it to the cart, would get a message, "We're sorry, we are out of stock on this item....(or someone just added the last one to their cart, click here if you want to be notified if they do not complete the purchase"....

Just thinking out loud too....