Interesting idea. However I suspect that in such a secondary market most liquidity/volume would only exist last minute, making it less likely to improve yield optimization (for airlines) or consumer choice (for passengers).

It's baffling to me that the overbooking problem is solved at the airport, minutes before take-off.

Think about how excess seats usually come about: a passenger doesn't show up because of some unforeseen circumstance which occurs last minute. By the time that passenger decides they can't make the flight, it might be too late to generate sufficient demand for that ticket. Potential buyers of that ticket have to be packed and ready to go. And where do we find those people? At the airport.

The population of people who are super flexible in both destination and time is probably not sizable enough to generate enough volume in a secondary market.