No Man's Sky

I found the easy money is to get the coords for a planet where a bunch of people have set up activated indium (AI) mining operations from the coord subreddit. Then I just travel there, set up a base for ease of access, and switch my network activity off. Every mine on the planet will be full of AI that you can freely take with no risk of stealing from the base owner since you're offline. Grab as much as you can fit in your inventories, then start flying around to find a good place to sell the lot.

The last step tends to be the tricky one. AI is a commodity that responds to surges in availability, so anyone who sold near you or near the mining planet will drive the sale price down for a while. If you find a system that you can sell the AI at market value, then you can make hundreds of millions in one fell swoop.