What happens to a listing that leaves my feed?
It is marked inactive: visitors no longer see it, but it is never deleted, and it becomes active again if it comes back.
After each complete read of your feed, a listing that is no longer in it is marked inactive: visitors no longer see it on the portal, but it is never deleted. If it comes back into your feed, it becomes active again. A listing your feed marks as sold is taken off the portal the same way and counted as sold.
If your feed suddenly has far fewer listings than before, or cannot be read, nothing is marked inactive that day, in case of a temporary fault.