As a fellow first time dev, I want to congratulate on your first game. I think you did a great job within the confines of a 4 day jam, and you have a really cool premise that you have a lot of opportunities can build upon, if you intend to update this game after the jam.
I think one of the easiest ways you can add a bit of variety to the gameplay is by fixing what I assume to be a bug. The number underneath a commodity name in the shop screen, which I'm pretty sure is the amount of the commodity available on that planet, is able to go into the negative, as shown below.
I think that if there were a limit on the amount of any given commodity you could buy on a planet within a short timeframe (I imagine the planet inventories could restock at the same time that the commodity values change), that alone would be a big improvement, because it would force the player to plan the most efficient route between several planets, rather than flying between the same two planets until the commodity values change.
Aside from that, I believe that, as other have pointed out, the game could benefit from adding something to do in space between planets (My mind goes to pirates that attack you and try to steal your cargo), and also I think a slightly more detailed UI (Maybe showing the values of commodities on planets, rather than just whether they're up or down [Although that may make the UI too cluttered, like I said, I'm a first time dev too, so I'm hardly an expert]) would be beneficial.