Some sites, like theater ticket reservations, place a limited-time hold on the item , while the order is being processed. As soon as you select the seat and proceed to pay, Seat is reserved say for 10 minutes.
I see here a big risk where a hacker/automated script(bot)/manual user can lock all inventory and does not actually make the payment. He keeps on repeating this process. This way seats will never be booked and business will loose the revenue.
How these sites handle this scenario/challenge ?
I really have no idea what various sites do, but its easy enough to dream up an algorithm to make it hard for fake customers or bots.
Include a test that most AI's find difficult (like reCaptcha as Robert Harvey suggests). This should eliminate the bots.
Then a procedure something like this...
Create provisional order for "Item I" with name/IP of person ordering.
Request Person A to complete payment.
If person A pays on time,
Remove item I from stock and empty the queue for that item. Inform queued customers that item I is sold and offer alternatives.
If Person A did not pay on time.
Go back to step 2 and start processing the next person in the queue.
If there is no demand for Item A, then who cares if mean old Person A likes to waste his time re-ordering and not paying!