Selling the Sentinel package will be far from straightforward. I suspect the US DoD will have a direct say on who they let operate the radar system. I'm not sure they will care how much money is offered for it; they would much rather control who gets to see their technology.
A possible outcome will be selling the airframes without the mission radar or system LRUs. A potential purchaser could then procure/install their own package. Several Global Express airframes are now used in a variety of roles by various military outfits. I'm unsure of the waiting time for new airframes.
Apart from the structural modification to fit the underbelly 'canoe', the Sentinel airframes are all very high hours (were and probably are still Global Express fleet leaders) and with a very high ratio of landing cycles. Furthermore, they have been superseded by the Global 6000 with its somewhat better upgraded cockpit.
Notwithstanding the continued problems of mission system reliability, the Global Express airframe was and probably still is the best available for such an ISTAR platform.