You're right, the trade currency of choice is Time Elixers and Potions (not the blue ones, those can't be traded). They're consumable items that can be used in events, but they also serve as the currency. It's unfortunate that you just barely missed out on the freebies, but there are lots of players who would be happy to help you out with some common units to get you started. The catch is that you need to be level 31 (player level, not card/unit level). To reach this, you'll need to do the Training activity for a while. You aren't that far from it, so it shouldn't take too long.