Is it currently legal to just sell your DIY power supply to US consumers? Or is there still like FCC certification required? Assuming, say, a USB-C charger.

"It depends" mostly on how DIY it is. PCB + parts kit is usually fine for most things that would have FCC or UL requirements, but selling something that's "assembled in USA" like tractors (wheels bolted on here, everything else done there) would probably still need certified.

The problem is design and tech companies and cloning resellers sling stuff direct to consumers via ungodly amounts of facebook ads. Because they’re direct shipments so many of those products are not safe or certified and many clones are labeled as certified but aren’t because they’re cheap knock offs made to look like the originals.

Even at the small end, it's hard to produce something that's even a bit popular and not have it cloned/ripped off/your chosen license violated. We have that issue with a number of our products, both open-source hobbyist stuff and non open source.