In fact, if you sell 10 hobby prototype boards to other hobbyists, you probably have a higher risk of getting hit by a bus on the way to the post office than you do of being fined.
Because there's a handful of companies (lectronz, seeedstudio, ebay, etc) that have the full shipment lists. So the compliance agencies don't have to go after individual business records one business at a time; they can go to probably fewer than 10 businesses and find the suggested mass rule breaking with a sql join.
See also the national tax agencies with NACE codes (euro version of NAICS).