Good luck to whoever has bought them - like a boat, the cheap part is buying the thing in the first instance. Hand in pocket constantly from thereon.
Entering into the realm of postulating, if they are headed to a TR it would be hard to think why someone like 707 or SRV would want them.
Whilst SRV have 500, 600, 700, 702, 703 in service, i guess they also have 711, 751 and 782 which havent appeared to not have been restored (open to correction), plus a fleet of SOP cars, BZ, W and E cars - hard to imagine why they would want more, although 502 is a beautiful carriage inside.
Probably the same could be said for SRHC too, not sure why they would want them, again more rollingstock just means the same bucket of money is spread thinner. Having more carriages doesnt mean you earn more income.
If they are going to private car owners, I hope they've done their research.
In any instance, its certainly better than them being ripped off their bogies and being used for some ridiculous tea and scone cafe in the middle of nowhere in the country.
No idea where you'd get spare BG bogies for the 500 and 600's as they are quite proprietary. Even the 700's arent there arent that many of, but granted more prospects than other.