They're trading for things that are still valued in an advanced society.

Obvious things are fur, spices, foods, wines, timber, wool, gemstones like amber or opals, pearls, crafted objects such as art, musical instruments or even furniture and pottery.

Sure most of these could be easily made by robots in factory but hand crafted items are worth a premium.

Now the traders are swapping cheap mass produced stuff they buy for next to nothing for items they can resell for a premium.

A robot manufactured stainless steel knife would be worth it's weight in gold compared to the hand forged knife made from impure steel. Ditto for cookware.

A extremely durable cold weather, water proof jacket is far superior to furs or woollen coats.

Pain killers and other medicines would seem miraculous to a primitive society or if the traders are assholes, they could sell illegal drugs.

Both parties would feel they're getting the best of the deal and trade happily

Like all traders, their long term goal is to get rich. I would imagine the traders found some backwater civilization and are keeping it a secret in their own society so they can cash in. They sell their stuff quietly and refuse to say where it comes from. Quite possibly, contacting primitive civilizations is illegal so they keep quiet about everything on both sides.

They're trading for things that are still valued in an advanced society.

Obvious things are fur, spices, foods, wines, timber, wool, gemstones like amber or opals, pearls, crafted objects such as art, musical instruments or even furniture and pottery.

Sure, most of these could be easily made by robots in factory, but hand crafted items are worth a premium.

Now the traders are swapping cheap mass produced stuff they buy for next to nothing for items they can resell for a premium.

A robot manufactured stainless steel knife would be worth its weight in gold compared to the hand forged knife made from impure steel. Ditto for cookware.

A extremely durable cold weather, water proof jacket is far superior to furs or woolen coats.

Pain killers and other medicines would seem miraculous to a primitive society, or if the traders are assholes, they could sell illegal drugs.

Both parties would feel they're getting the best of the deal and trade happily

Like all traders, their long term goal is to get rich. I would imagine the traders found some backwater civilization and are keeping it a secret in their own society so they can cash in. They sell their stuff quietly and refuse to say where it comes from. Quite possibly, contacting primitive civilizations is illegal so they keep quiet about everything on both sides.