If one is interested in a machine, one can be made with a radio, but that does not mean there was not a collective culture in tattoo machine selling to try to keep machines in studios.
As for quality, in my experience cleanliness and safety could not be guaranteed in a shop even with the certificates and licenses.
This as well as the safety thing. Cross contamination is no joke! Also, a setting that isn't luring sexual assault victims to a private residence. A front facing shop lends accountability in a few fronts, like taxes and comradery between shops, too.