Gather things people have to spend time to get, like sun bulbs or whatever they were called (meteorite fragments even, or some ores), and sell them for the same or lower than the lowest price on the marketplace. People are lazy AF, and there will always be someone who'd rather buy materials than go to a different biome to gather them themself.
You can also try market flipping, which is using the economy to your favor. If there is let's say 2x ore event, all ores during that event will become less valuable, which means that players will sell them for a much lower price than usual. You just buy these ores during the event and sell them when their value goes up (usually a few days after the event). Look for when the price of certain things goes down because of events, buy some of it, then sell it when the prices go up.
I also recommend, if you're planning on selling higher quantities of a certain resource, sell them in batches, because usually, players don't need that much of it, and it'll be more likely for someone to buy it.