honi, bunkboy is surely right. It's a cash business with limited documentation and that's the way the proprietors like it . Once they come to the notice of the local revenue authority, they'll receive a visit and their bookkeeping will improve .

I expect that, as in Europe, the authorities have a power to assess tax unpaid. If the methods of the authorities in Britain and France are anything to go by, the 'assessment' is very generous to the authorities, enforceable in full immediately as a debt and no rebate or recalculation is made unless and until the business produces full and satisfactory accounts. There are often penalties and interest in addition to the assessed sum, too.
So one 'phone call from you to the local revenue body should do the trick. That's the right thing to do, because this business is in unfair competition with law-abiding businesses and increasing the tax burden, by however little, on the rest of the taxpayers.
It is quite likely that Uncle Sam does, in some circumstances, give tax breaks to people setting up businesses. He may do so to encourage the revival of old industrial areas where the local industry has died out or to create new employment in some region. He may do so to encourage foreign investment by corporations,with, or more likely without, specifically subsidising them because they are foreign. (That's how the European Union, and before it Britain alone, has operated and explains why of our old coal and steel areas are populated by Japanese carmakers and American computer firms

)
Somehow I don't think a Vietnamese couple with a second-hand business would meet any such criteria

.