I have a boat in charter with the Moorings in the BVIs and have looked into both seling it and running with it, so hopefully I can shed some light on some of your questions.
Most boats chartered by the Tui Marine
services, (Moorings, Sunsail and Footlose) are owned by private owners. A brokered sale
would therefore be much like any other brokered boat sale and is negotable. I think the difference is that there will be many boats of the same age, same make and same outfitting all for sale
at the same location at the same time. The result is that there is a more defined market price and fewer owners who have outrageously high asking prices. When I talked to a broker, I observed my model boat at least tended to have asking prices about 10% above what they were selling for at the time. There is wiggle room but I think you will find it's more defined than with other used boats for sale
Many of the Moorings boats are U.S. documented. Mine is. I can't speak to the process for registering foreign documented boats, but I'm sure they can help you out as that must happen all the time. I think where you are more likely to get nailed (and where I may get nailed) is with the state you bring the boat back to and their state registration process. They may tax you on the value of the boat. Obviously, the charter boats, won't be registered in any U.S. state.
If I keep my boat, I'll probably sail it back to the U.S. myself. Should be a fun ride with the currents and prevailing winds.
As the Zanshin said, the charter boats for sale usually don't come equipped with SSB's, dinghy's, etc.
Check into insurance
My experience is that charter boats get hard use, but they often don't have the neglect and jury-rigged projects that many boats for sale by non charter owners have. They needed to be maintained to charter-ready standards. It probably has more engine
hours than someone who ocassionally day sailed, but they often put a larger engine
in the boat in the first place. Different trade-offs.