In addition to selling goods for coins, coins are quickly earned from hosting visitors as well. For example, see the Express Starport Atlantis achievement where 1000 coins must be earned in a short time.
Unfortunately, completing the Starport Atlantis challenge will not count for the Capitalist achievement.
As stated by Albert, above, build a Landing Pad (even better, build multiple Landing Pads and Starports) to trade goods and host visitors for coins.
One note: When trading goods, the trading ship will have a limited amount of coins. Don't give them additional goods for nothing.
Hope this helps.