Most traditional Superman methods of getting money are much harder or even impossible now. You can’t just walk into a jeweller’s or brokerage in most modern countries and dump a handful of diamonds or a lump of gold on the counter; it’s basically valueless without provenance. You might as well try and sell a lump of weapons-grade plutonium. Those who would take a chance and pay you for them would give you less than a cent on the dollar.
If you obtain and try to sell manufactured goods such as trading cards, they’re traceable - how did a rare card from China end up in an eBay auction in Germany without any obvious transit? You might be able to sell to unscrupulous billionaires and similar who don’t care about legality or provenance and who can and will pay for all sorts of stupid stuff, but that’s ethically very dodgy and they will probably still track you, out of curiosity if nothing else, and you got more troubles. Heck, at that point you might as well try and sell cocaine you confiscated from drug cartels.
There are simply lots of things you don’t need money for (travel, for instance) and maybe you just build safe houses with your bare hands in extremely remote and inhospitable regions. It’s illegal because you don’t own the land, but it’s less problematic and harmful than many other options.
