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He bankrupted 3 casinos because he didn’t understand the simple business concept of flooding a market.  He opened one and it was a success, so he opened two more, with pretty much the same concept less than a mile from each other in Atlantic City.
They basically cannibalized each other’s clients and he went from having one full casino to having three mostly empty ones.  For those who say he was not using his money and just fleecing the investors, some of that is true, but Trump tried to make the casinos work, so much so that the he tried to make the last one work for decades despite years of loses and finally shutting it down in 2015.
Furthermore, he kept trying to make his casino work by opening another one that also failed in Gary, Indiana.   
Trump loved the glamour of owning a casino, he was just terrible at running them.
Didn’t need his tax returns to know he was dumb af and shitty at business.