The flip side of strong ethnic ties and ingroup preferences enabling fraud, is that they make it possible for, eg, Hasids to conduct an extemely high-value diamond trade on a handshake basis internally. https://www.algemeiner.com/2014/05/21/new-yorks-diamond-dist...
I don't think that's true; traders in the stock markets with no racial or familial ties regularly make such deals, especially in the days of open outcry markets.