Looks like there's some confusion in this thread between Black Market and Grey Market goods and practices:
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Grey market goods and practices are legal, but the usual distribution channels are not used which allows sellers and resellers to make their profit. E.g. if you buy goods on Ebay USA, import them, pay your duties and taxes and then resell the goods on Trade Me then that's the grey market. It's completely legal and a lot of people make their living this way.
Black market goods and practices are illegal, e.g. goods = narcotics, human organ trafficking, fake designer goods... practices = smuggling, avoidance of tax and customs duties, etc...