Star Trucker

This is a little ambiguous, so I confirmed with developer what the requirements are:

"I've just double checked. Selling goods that "could" be restricted ( e.g. fireworks ) at any shop that will accept them counts towards this achievement.

In hindsight we should have made this a bit trickier and clearer in the achievement description 😅"

They also said;

"In the future we hope to revisit this area of the game to make it a bit more understandable and engaging. Potentially alongside a more nuanced galactic economy pass."

So to unlock this achievement you need to sell any of the goods that *can* be contraband to any market that can take them. Potential goods are; alcohol, explosives, narcotics.

All goods can be sold at the various black markets.

But it's worth noting that not all goods are illegal everywhere, hence the ambiguity. If you can find a market that sells, for example, alcohol below average, and another that buys it's legally above average, then that would be a valid contribution to the achievement.

Illegal goods are also found in salvage beacons (which are easier to find once you've progressed Moon Baby's story, once you get a truck upgrade.)

Illegal goods are easier to trade in when you progress Barrow's story to the point where you get a truck upgrade.

People have laos said they've simply purchased illegal goods at a shop, moved them out of the airlock and old them back to the ssme market immediately. This should work too to be fair.

"Buy low, sell high"
This game incorporates a system very similar to Drug Inc. from way back on Newgrounds.com.
Each sector has higher or lower local prices for every product available at supply stores. These percentage changes can be found on the map. Purchase things that are on sale, then head to a nearby sector where the prices are higher, and sell. This can be planned in conjuction with transport jobs, by checking the route and stocking up before hitching to the cargo.
The same principle will work for the traders achievements.