Architecture guidance on realtime reservation payments - Paypal, Google CO etc

I am building a real-time reservation system and would like to allow users to buy a specific time slot using Paypal.

I can use IPN to call back to the server once payment has been completed however I am unsure how to proceed if the slot has already been sold i.e. another user has bought the slot moments before. At this point payment will have been taken.

I could potentially "lock" the slot as the buying process has been initiated however what happens if the user starts the buying proce开发者_StackOverflow中文版ss and then quits half way through?

Can anyone advise on a stratergy for such payment types?

Best regards, Ben.

You were correct, you put a lock on the slot but let it be time-based expiry. So if after a certain time that slot is still in locked state free it.

States assumed : FREE, LOCKED, ASSIGNED