If Zillow could conceivably have enough inventory for both sides of the transaction, that would be a real win/win. I.e. not just buy but also have an upgrade/downgrade new investment property.

Me thinks they would need thousands of properties to make that happen but they could go through a prearrangement process. i.e. if we find something, then you could sell and buy.

In Russia, years ago property transactions could take months, as the buyer would usually need to find a buyer for his place etc, I heard of buyer chains of upwards of 5-7 transactions involved.