I'm a subscriber on /r/warshipporn and it seems that every 10th ship or so is a ship that used to belong to a previous nation that is now under the ownership of a new nation.
Did this only become common after the creation of large warships or was it common to find outdated swords, or armor in the hands of allies as it was replaced.
Absolutely! I'm not an expert on the subject, but a big part of it was that both the USA and Britain produced vast numbers of ships for WWII, many of which were simply not needed after the war (smaller wars) or were made obsolete by post-war changes in technology (battleships obsolete, rise of rocket tech, ever-larger airplanes neccesitated larger carriers). A number Latin-American countries continue to operate WWII-era ships that the USA either outright gave away or sold for a song (better than scrapping).
Try messaging /u/an_ironic_username, he's our resident expert on 20th-century naval warfare.
Absolutely, this dates back as old as Sun-Tzu. Sun-Tzu believed in the "spoils of war" i.e. sacking and pillaging your enemy for your Army's gain to relieve strain off your own economy. If you think about it is a very smart tactic. Sun-Tzu thought about in terms of this, if you are campaigning into a larger country, progressing forward, back in those times technology made it a logistical nightmare. Why not use what is already there instead of wasting weeks and months on ships and horse drawn cart supply trains to reach your troops, especially if they moving faster than your logistical/supply train can move. This is the exact issue General Patton and the Third Army faced in WWII.
The second point is that not only would this contribute to your country's/army's personal gain but selling enemy technology would also allow you to recover faster from casualties, loss of weapons, clothing, food, etc. Basically, everything an army needs to fight.