First of all, welcome back to the game.
1. To gage the price of ectos the best way to start the day is check the Rare Materials Trader AI located in most guild halls or Embark Beach or any major town. Kamadan traders should riff off of this price and sell slightly under it.
You can sell it to an AI Rare Materials Trader but it will be worth far less than his sell price. Better to sell to people.
2. International Zone was never remotely the most active zone. Since the very beginning the American Zone had the overwhelming numbers. As the game aged and the numbers dropped it still had the most numbers so more and more people in European or Asian zones simply moved over to it.
Int'l Zone originally was created more as a meeting place to party up between players playing in otherwise different languages and time zones and never was a major trading zone per se. In fact many people switched to the Int'l to avoid the endless trade chatter in Kamadan when wanting to play Nightfall in peace and quiet.