Not to derail this thread too much, but the warning I gave was about making and selling aircraft parts. The FAA frowns upon this practice. Their job of catching people who make unapproved parts is made easier for them by simply looking at eBay or similar marketplace or website. The fines can be hefty. Sometimes jail time if it really affects safety-of-flight. Typically the FAA will then look at the maker's customer list and send a warning to everyone who purchased parts to remove them from their aircraft.
We can debate about whether or not such parts are safe or good, and whether or not to use them. That's for the end-user (and/or their mechanic) to decide. I'm only saying that making parts to be sold on certified aircraft is not as simple as printing them on a 3D printer and selling for big profit.
Here's the latest example I can find:
Press Release – FAA Proposes $203,100 Civil Penalty Against GEE-BEE Aeroproducts [
faa.gov]
