How about instead of deserting to another fam, they end up on a 'merc market'.

Better would be if a 'merc market' were integrated into the main market.

The GC from the sale of deserting fleet would go to the player whose fleet is deserting, thus helping to upkeep the player's fleet.

Another option is that deserting fleet could desert into the player's empires' population - this would be helpful for pop bankers who have gotten their pop blown by ops and don't have the GC to upkeep their fleet. 

Intentionally disbanding fleet would also replenish pop / get placed on market.

First, disbanded or deserting fleet replenishes an empire's pop.  Second, once pop is maxed, deserting or disbanded fleet that does not max-out an empire's pop in a given tick is automatically placed on market.  The disbanded or deserting fleet would be automatically placed on the market at the market's current highest buy bid price.

Players would also have the ability to manually place fleet on the market at a sell bid price of their choice if they want.

Last but not least, the whole market system should be changed so that you can trade any resource (including GC and fleet units) for any other resource.

For instance, say a player had a whole bunch of food, no GC and desperately needed iron.  Instead of waiting for his food to sell so he could then buy iron, he could put a bid on the market selling x amount of food for x amount of iron.

Likewise, maybe another player had way too many agents and no GC to upkeep them.  He also wants to build figs to station on an umpire allied planet in an enemy system.  He does a bunch of ops, dabbing a few portals, then places a bid for x amount of agents on market for x amount of GC.  The price is cheaper for a lot of players who might want to build them themselves AND who lack the endurium to build them. 

So the agents sell quickly, and the player now has the GC to build his figs.