Bought one myself for EC a few days ago after a couple of weeks grinding EC. Here are a few tips.
Lock box keys carry the worst CP to EC ratio. The best is the uncommon crafting material which is purchased at Memory Alpha for 1000DI. Sells on the exchage for 70,000 EC each. You can push for a little more by making antiproton consoles, but the market really does vary.
Buy the EC credit upgrade if you are silver. You'll need it
Doff selling makes a nice profit, get all your characters running doff recruitments as often as you can.
Dont be afraid to dabble in the exchange, there are several items where the price fluctuates. Buy low sell high.
Run the tour the universe event, nice EC reward, plus you can do some doff missions on the way round.
Be tight, dont spend ANY EC at all. Just have a little patience.