I really want to see the market running. I don't know what I would buy, probably planets although planets where you have only one far away from the rest of your system I think are pretty useless. You would need to buy a planet closer to your other planets or buy a set of planets that are together.
Another buy that I will be willing to spend a few dollars will be big ships to protect the planets that I already have. I few days ago I was the victim of an attack and I was unprepared. I was able to prevent the lost of resources by spending them but I wish I could defend my planets and in some cases get revenge of the players that stole the resources from me.
When the market is running, what do you plan to buy?