So all plumbers, carpenters, electricians, Teachers (who do grinds), gardeners, painters, stall owners, shopkeepers, solicitors (who accept cash payments) etc in Greece are rich.... wow, what a great country.
Some are, yes, depending on how much money they bring in in the first place. Longer hours=more money. The point is they do not contribute to the economy leaving the tax burden to the PAYE sector.
Shopkeepers generally pay tax because they are required to give receipts and there are loads of spot checks.