Unfortunately, that will not fix anything. Letting your shopping cart's session expire is no different that posting "releasing dibs". The people hurt the most by the retracting/releasing dibs concept are those that already saw the dibs, and didn't give the thread a second thought. When the item is available again, they never know.
One solution that comes to mind, is for a classified ad owner (by way of a new mechanism on the website) to send a specific kind of alert to interested parties when a dibs has fallen through. A way to be notified when items come back up for grabs. One might say we already have that through "backup dibs", but as
@ranger764 pointed out, sellers can make mistakes when looking through the list for backups.