As an extremely-low-frequency user of Coinbase's services, I personally wouldn't really care if, when they make necessary crypto trades to liquidate commissions or maintain requested customer/index-fund holdings, they used their proprietary position to get a slightly-better deal.

They have to make their profits somehow. And, the ultimate check on their profits is competitive offerings. Thus a slight profit here may just help them charge less elsewhere, in the fees I explicitly pay. (I can see how a professional or high-frequency trader would feel differently, though.)

But, a further clarification from Coinbase has confirmed my skepticism of the nefarious interpretation of the "20% volume" statistic. The 20% figure s driven by trades at the direction of of customers, not for Coinbase itself. See:

https://blog.coinbase.com/correcting-the-record-coinbase-doe...