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In order to answer this question we also need to think about industry.

What can be manufactured?
How easily can it be manufactured?
Are there any scale efficiencies? Does the cost of producing a product lessen drastically when you increase the size of the manufacturing process? (eg. at one time publishing was dominated by large publishing companies that used huge presses, but now we have digital printing, so print-on-demand is cheaper unless you are printing something in very large quantities.)

Realistically in any space opera game, very few raw materials will be interstellar trade goods. Although you could always invent something, unobtainium but with a less insulting name.
In a highly developed nanotech world, only information will be an interstellar trade good.
With less nanotech, organic luxury goods would still be interstellar trade goods. (eg. real champaign, Greek olives, Ethiopian coffee, Chinese tea, etc.) Some organic luxury goods could be interstellar trade goods twice, like chocolate. I'm sure Dutch chocolate is grown in some tropical place, brought to Europe to be made into high class sweets, and then shipped out to still other places.
Even with little or no nanotech, certain items might become so easy to produce that they are always made locally. If computers in your setting are gold ink printed on cellulose sheets and then glued together to make sandwiches, maybe that kind of technology exists at anyone's desk, so computers, AI, robots, all communication tech, etc, none of that would be trade goods.

All good points above but lets look at Earth trade. It would seem unlikely that it would be prohibitively expensive to ship shoes half way round the world to sell them someplace else. Even less likely for a cheap product to be shipped multiple times around the globe just to sell for $5. but that's the economy we live in now.

Transportation is cheaper than the labor difference between two locations and that means transport is economical. So there would have to be a galactic powerhouse economy (or several) where huge populations live and consume products of different planets.

Alternatively if a planet like Earth were no longer able or allowed to grow food then imports would be important. Maybe the PETA party passed a law that made it illegal to grow animals for food but they couldn't get the public support to ban meat consumption and so if someone wants a burger cows have to be flown in.

Conversely a planet being terraformed could require large amounts of materials and food to support the work. That could be a big source of trade.

It all depends on the tech level of the civilizations and what is cheap to produce. Maybe laws prevent a cheap manufacturing technique because it's polluting but a colony world with no breathable atmosphere can use the technique and doesn't have to worry about it.

greenhouse gasses on mars, would be a good thing!

If you want to do some "research" look for a game called Escape Velocity by a company called Ambrosia. It's all about trade and 100 ton cargo ships. Seriously one of the best video games ever. Really simple overhead graphics but play is really fun and slowly addictive. The current version (Nova) is available on windows but there were two others that were only available on Mac but you can skin the Nova engine to play the old games. Thats a little more complicated though.