It seems to me that the better choice to creating a new problem to solve an existing problem is just to get rid of the change that created the existing problem.
There was nothing broken, IMO, in having few capturable survivors of a gunfight between two gangs. I would agree that the system needs to respond differently if the game suddenly provides an influx of slaves, but that should be in the form of a market system (supply and demand curves), not arbitrary market collapses. Having a market system is probably on the list of things to do, but probably way down the list. I think that making all opponents capturable should wait until the game is ready to "realistically" (i.e. something the players can suspend disbelief for) portray the impact of many new slaves on the market. Though, tbh, even selling 8 slaves on a given day is not going to have much lasting impact in a market this large.