A friend of mine used to have a small side business selling loudspeakers for medical equipment. She order them in Asia and had her own little certification lab at home: heat chamber, impedance testing, that kind of stuff. Every so often, she’d receive an order of a few hundred speaker, and test them one by one.

It’s a low volume but high margin business. Some of the issues were the constant fight against the factory not following design requirements to cut costs, knockoffs etc.