From an economic perspective, I wondered why this idea took so long to implement. If United could sell my seat for $1,000 (that I paid $200 and was flexible with my flight), and "share" some of the increased revenue, it seems like this would be a "no brainer".
In fact, I would go even further to identify and maximize the economic utility of other areas. For example, an upgrade. If I've been upgraded to first (or business) class, would I will willing to "sell my seat" to go back to coach? Of course, there are numerous factors and everyone is different, but at some price, this would be attractive.
If another passenger was willing to pay $1,000 to upgrade to business class, and I was willing to take $500 to go back to coach, wouldn't we both benefit?
For those passengers who have upgraded, perhaps there can be a field where they would be willing to volunteer to be downgraded to coach (and how much it would take to do so)? This would be especially helpful during irrops where First class passengers would need to be re-accommodated.