If the project is actually just a hobby, then CE and probably FCC testing is not needed.
If the project is a startup, then the cost of labor is not zero, at least not if people are not deluding themselves (i.e. at least opportunity costs should be considered).
Personally, as a consumer, I'm pretty happy that I can buy e.g. a wireless mouse or a bluetooth speaker and can reasonably assume that they actually work and aren't accidently jammed by some "startup"'s hardware.
> If the project is actually just a hobby, then CE and probably FCC testing is not needed.
That's the entire problem: According to the law, it is! There are extremely few exception, and the exceptions that do exist are essentially useless for hobbyists. Everyone selling small-scale prototypes on websites like Tindie is just rolling the dice and hope they don't get get a life-ruining fine.
There are plenty of $5-$50 trinkets I'd like to design and sell as a hobby to fellow enthusiasts, due to their niche nature probably only a few dozen of each. But there's no way I can afford a $5000-$10.000 testing & certification fee on each one of those, and without that I'd be breaking the law.
You can sell kits, because companies do. Kits are only required to be authorized in very specific rules for specific types of kits. However, devices assembled from kits are not exempt from FCC authorization requirements. Home built devices not assembled from kits do get an exemption. You figure out the contradictions here - i.e. this is not advice.
The FCC has fined people for assembling and selling uncertified radio transmitters from other's kits. Like this guy:
https://docs.fcc.gov/public/attachments/DA-12-574A1.pdf
Note that the company that sells the kit he was assembling is still selling the same kit that results in an uncertified AM-band transmitter when assembled.
If the project is a startup, then the cost of labor is not zero, at least not if people are not deluding themselves (i.e. at least opportunity costs should be considered).
Personally, as a consumer, I'm pretty happy that I can buy e.g. a wireless mouse or a bluetooth speaker and can reasonably assume that they actually work and aren't accidently jammed by some "startup"'s hardware.