After you've collected some Wyrdstone (Green rocks) in a mission, the smuggler's den will be available. Ship some Wyrdstone to a buyer, then wait a few days for it to arrive. You'll get credit the day the shipment arrives.
The fastest way to earn this achievement is to ship one piece of Wyrdstone to each buyer, each day. You can't make multiple shipments to the same buyer in one day, but this way you can earn three credits each day.