For an AU the important part is . . . what year is it? and what was the timeline branching effect?
You have two different pools of buyers IMO- the Inner Sphere and the Clans. The Clans? Yes . . . you could sell to the Diamond Sharks, Warden Wolves or the Nova Cats if you were willing to accept payment in kind- spare parts for the one you are keeping, upgrades & additions to your ground forces, and IMO even dropships. You would not get paid as much for the ship by a Clan source just because heavy capital ships are not as rare among the Clans- Leviathans, McKennas, Nightlords, Texas, Camerons, and Black Lions are all easily found in Clan fleets.
Among the IS . . . it is like owning the Mona Lisa or Rosetta Stone . . . no other exists, nor were there peer replacements. The Mjolnirs come closest (but still lose, and might not exist in that form based on your timeline) but only two exist and both belong to a single power. The FWL fleet can match the firepower with numbers in their Thera TFs . . . the Dracs, FedSuns, and Cappies would have to gather most of their fleets. Liam has some good points, but for a price I think he is off by a order of magnitude for the value . . . this is not like where a mercenary brings in salvage and gets paid 40-60% of the MSRP value to a merchant who marks it up after fixing it up to MSRP + 10-15%.
I mean you could probably ask Hanse for your ugly hunchback daughter to marry Victor and the McKenna is the dowry.
The Dragoon's fleet was home ported out of their own private world and thus sort of stood apart . . . but it would be surprising if a House did not have a restriction on what private warships could mount for weapons. If you mercs are based in a House's space, you might be forced by law to strip the weapons down to a light cruiser equivalent based on some earlier laws. After all, a House is going to want to outgun mercenaries and that law would at least go back to 1SW when the Houses actually had heavy capital ships of their own.
I would also point out, the only warship outside of House, Clan or Dragoon hands? It was repaired at the Warden Wolves' Lupus Majoris . . . at least what repairs the Blue Star Irregulars could afford. Particularly important to you, the facilities can accommodate a McKenna (Werewolf is tended at the station), the crew/staff has the only experience outside the Clan OZs with a McKenna (btw, NONE of the OZ Clans actually has one after the Jags lost theirs), and they can source parts to repair warships- maintaining the Wolf fleet, repairing the Black & White Paw (Aegis class), and fixing up the Winter Wolf/Yggdrasil.