You can't really smuggle a friggin battleship, and those things are not something you can make in your own garage.
Size is rarely an obstacle to whether or not something can be smuggled. Where there's a will, there's a way.
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Ok, how would you smuggle a Nimitz carrier?
Because surely, any produced unit is tracked and has a serial number. They are far too big to pass undetected, and far too valuable to simply ignore when one goes missing. This isn't a small arms shipment that goes missing (and even that get heads turning).
You can't approach any port with it. In SS terms, with a stolen battleship you wouldn't be able to approach a planet.
Any ship of that size will HAVE to be registered. And yours either won't have a valid registry or you will have to use a fake one and that is not bloody likely to work, when every single ship will be accounted for.
An Onsalught with a serial number SN-ons000075 would have been registered with the hegemony 5th fleet, under capt. Thorne. You're clearly not Thorne, and that fleet you're part of clearly isn't the 5th Hegemony fleet.
So even if you COULD steal such a ship, you would not be able to sell it or buy it in normal places.
A secret ship dealer somewhere in hyperspace or on some remote rock, perhaps.
The only time military warships are sold are in 2 cases:
a) to other countries and governments, and only friendly ones that are considered no danger
b) to private individuals/companies, usually lower-grade stuff or completely stripped hulls with any sensitive equipment/weapons removed. The russians sold one of their old aircraft carrier to be turned into a hotel/casino, IIRC.
It might be interesting to have rare "auction" events pop up as random missions. They could provide a source of rare items like blueprints, weapon caches or even exotic/rare space ships. I don't think anyone would really ask where the rare things came from or why the serial numbers are filed off, but there will always be buyers for such curiosities.
The "auction" mechanic allows rare things to stay rare, but at the same time the player isn't stuck with personally scouring every single planet for rare deals every time the market changes.
This is actually a good idea.
Such auctions could be held by different factions, with different things offered depending on your standing with them AND their general stance/ideology. A highly militaristic faction would restrict higher-level weapons/ships, but would offer lower ones in abdundance. Pirates on the other hand might be willing to trade anything, but their offering would be mostly salvaged ships and truly good ships would be rare. Corporate factions might be willing to trade more advanced things compared to militaristic factions, but they would also rip you off.
Getting access to such auction would require a bar event and good standing with the faction. They could be held at a randomly selected location.
If you want to go even deeper, the merchant fleet could be generated that would carry all of the hulls, and it would wait at the location together with a few minor buyer fleets. You could even try to attack and rob the gathered, but that would loose you invite to future auctions.