I suspect a bit part of the reason they kept the oafan ships rather than selling for mad stacks of cash would be time and availability.

Petey 1.0 was a fluke, a properly powerful military ship isn't something you can just throw a bunch of money at and drive away from a shipyard with. It'll take time for all the orders that were there before yours to finish, it'll take time to reconfigure between jobs, it'll take time to build the thing. Then add in on top of that not necessarily having access to all the shiniest hardware options because governments don't really like nongovernments having all the fun toys. Which admittedly is the sort of problem you can theoretically throw money at to fix, but still.

Plus, you know. Long Gun. And Kaff's long-established liking of extravagant ships.

Also, they DID sell a bunch of oafan hulls, that was literally (part of) the reason they went to Earth.