Most of these answers are found in the questions mentioned as possible duplicates, but I think that the following discussion still has some value.
First of all, trade requires different goods that can be exchanged. So instead of asking "What good would they trade?", the question should be "What goods or services would they bring, and what goods or services would we give to them in exchange?"
The simplest commodity for this situation is Energy. To quote the Hitchhikers Guide to the Galaxy, "Space is big, really big..." and to get from one place to another in a reasonable amount of time requires a lot of energy. If visitors were to come to our solar system with certain things we want (<PERSON> gives some good examples), they would come asking for energy. If they were able to load up with materials like hydrogen and uranium when they visited us, it would make a good stop, especially if they didn't have to mine those things themselves. The reason they would prefer to trade as opposed to mining themselves is that mining requires equipment, which means they have to bring that equipment, making space travel much more expensive.
There is also another advantage that Energy has; namely the fact that it is both exclusive and rivalrous. This means that it can be treated as private property and that when one person consumes it, others cannot. 159
These are almost essential to using them as trade. The issue with using information as a trading good is that it is neither of these. There is no way to control it as private property (without some higher governmental enforcement agency), and the fact that you had an idea does not keep me from having an identical idea on my own. Wikipedia has a good discussion of the topic, and you might be interested in doing some further research on the topic.
As a result it might not be that people trade knowledge per se, but instead basic resources (energy and materials to repair ships) and technology, or implementations of ideas. Maybe everyone knows that splitting water makes two hydrogen atoms and an oxygen, but if one species has figured out a way to split them with as little energy as possible, that is technology. That is something people might trade for.
In summary I would suggest that you study the following economic principles: Exclusivity, Rivalry, the economic concept of Utility, Opportunity Cost, and Comparative Advantage. These are the fundamental principles that modern international trade is based on, and intergalactic trade is just a much bigger version of the same set of problems and solutions. 207