Redefining Post Scarsity

It might also be that while the colony is post scarcity, individuals aren't. They get a fixed amount of income from their duties that they can spend, and they use trading as a means of getting more.

For example, in Star Trek Voyager, the crew can literally replicate anything that they could ever want, but not as much as they might want, due to power rationing. The crew would replicate things and then trade them up on local planets.

They might use their replicator rations to make a bottle of expensive alcohol, and then trade it on a planet for a much larger quantity of cheaper alcohol.

Your colonist might do something similar. They could make complicated machinery in small quantities, but what they might want was something much simpler but much larger that their production allocation didn't stretch to.

Utilitarian Future

It might also be that the colony's automated production facilities were locked down to strictly necessary items. They could produce an unlimited amount of practical machinery, but not a purely decorative item. So they trade for them instead.

Romulan Brandy

In Startrek the Next Generation, Enterprise crew members could get near limitless amounts of food and drink from the replicators, but it's commented on several times that what they get isn't real food. It's a nutritionally balanced substitute that's made to look and taste like real food.

It's also commented on several times in DS9.

Synthahol won't give you a hangover, replicated chocolate cake won't make you put on weight, and it never tastes exactly the same as the real thing. Which is why the senior officers always seem to have a bottle of contraband alcohol somewhere in their quarters.

You're colonists might be in a similar situation. They get supplied with healthy food, but they might want moonshine. So they trade for it.

Redefining Post Scarcity

It might also be that while the colony is post scarcity, individuals aren't. They get a fixed amount of income from their duties that they can spend, and they use trading as a means of getting more.

For example, in Star Trek Voyager, the crew can literally replicate anything that they could ever want, but not as much as they might want, due to power rationing. The crew would replicate things and then trade them up on local planets.

They might use their replicator rations to make a bottle of expensive alcohol, and then trade it on a planet for a much larger quantity of cheaper alcohol.

Your colonist might do something similar. They could make complicated machinery in small quantities, but what they might want was something much simpler but much larger that their production allocation didn't stretch to.

Utilitarian Future

It might also be that the colony's automated production facilities were locked down to strictly necessary items. They could produce an unlimited amount of practical machinery, but not a purely decorative item. So they trade for them instead.

Romulan Brandy

In Star Trek the Next Generation, Enterprise crew members could get near limitless amounts of food and drink from the replicators, but it's commented on several times that what they get isn't real food. It's a nutritionally balanced substitute that's made to look and taste like real food.

It's also commented on several times in DS9.

Synthahol won't give you a hangover, replicated chocolate cake won't make you put on weight, and it never tastes exactly the same as the real thing. Which is why the senior officers always seem to have a bottle of contraband alcohol somewhere in their quarters.

Your colonists might be in a similar situation. They get supplied with healthy food, but they might want moonshine. So they trade for it.