They traded with spices (Season 3) and with goods. They had to pay the automated repair station (I forget with what). I'd say they were paid, but we didn't see anyone really talking about it.
Perhaps that was a missing Season 5 story - Mayweather Gets Audited.
They traded with spices in S3 because Earth money would have been worthless to an alien living in the expanse.
They paid the automated repair station with 200 liters of warp plasma. Well, not really, seeing at they used the plasma to blow up the station, which subsequently and creepily repaired itself.