There are many third-party appliance/electronics parts companies.

Yeah - and the problem here is that there are many parts.

Getting a plastic wheel for my dishwasher's lower roller shelf is $12 from a third-party source (it's $16 from the manufacturer). It's like four grams of plastic. But the rarity of demand just doesn't make it make sense to keep in stock.

(That sort of thing has me halfway convinced to buy a 3D printer. I just don't have anywhere to put it yet.)

Make sure to check for generic wheels too! I think 90% of dishwashers have the same wheels but with different parts numbers.

Good shout! The wheels are largely interchangeable but the wheel mounts on the basket aren't (and one of them popped free and melted on the heating element just this week!).

true, however the current trend is to lock down the parts and prevent companies from making replacement parts via the use of DRM, serialization, and other controls that have nothing to do with security, usability, or anything other than preventing repair

Hell even Paper has DRM now, see the Latest Dymo label printers

How do these parts get recertification ie how do we know the anti skid brakes still work after using third party wheel speed sensor on just one wheel?

If the third party parts went through the same certification process, they would be more expensive than genuine parts.

It is based on specifications and standards. In this case the original part has some specifications that aftermarket parts can meet.