The trick to wash trading on CoinBase is (or was) to be on the inside:

> The order also finds that over a six-week period—August through September 2016—a former Coinbase employee used a manipulative or deceptive device by intentionally placing buy and sell orders in the Litecoin/Bitcoin trading pair on GDAX that matched each other as wash trades. This created the misleading appearance of liquidity and trading interest in Litecoin.

https://www.cftc.gov/PressRoom/PressReleases/8369-21

I wouldn’t be surprised if the wash trading on these unregulated exchanges followed a similar pattern where insiders were largely using the system to their advantage and using their insider knowledge or connections to (try to) hide it.