It is a little off topic but when is the first known warship sale by the Terran Hegemony to another party? Stated canon text please. It can't be the 24th century cause that makes no sense.
Well here is what we know
Vigilant (TR3057r p. 188) - In 2409 the TH began building the Vincent. "The vigilants were eventually mothballed or sold off to the various houses as the Vincents replaced them"
Dart (TR3057r p. 212) They served for 200 years before they were retired. Many were sold to periphery states.
Black Lion Is (per FMCC2765) were not sold to the Great Houses until just before the RW
The Aegis was not sold to the great houses until the RW per TR2750 & TR3057
Baron (TR3057r p. 200) were sold to the houses in 2720
Lola I - no data but per TR3057R p. 210 they served until 2622
Vincent - no data but I would suggest after the formation of the SL since the TH made Delhi build a new shipyard in the Delhi system to protect their technology
Cruiser Cruiser- mothballed 2405 " but several would get a new lease on life after being sold to the Great Houses" (FRDC2765) The date is obviously very vague.
Still I can only logically see such a move made in the course of the formation of the League and the end of the Age of War.
I can see this happening during the reign of Deborah Cameron and her policy of Aggressive Peacemaking. Which kind of fits with the sale of the Darts.
Two, when was the first Dreadnought Warship engagement and who participated in it? Gotta be in the 2300 s.
We know the FS and TC fought a battle in 2368 at Robsart (periphery 1e. p. 18).
The Cruiser Cruiser entry talks about it being used against other warships but with no details. Unfortunately details are scarce- If I can find anything I will post it here.