Pro miners doesnt sell at all. pro's doesnt concern about the bill. Then you are some kinda rookie daytrader that think you can make profit from small fluctations.
Pro means you do it full time. Most professionals work off a Profit and Loss. Expense is relative to your operational cost & your take home(personal lifestyle). Everyone has bills. And the most successful business owner does concerns themselves with bills and cost. It takes money to make money.
But yeah, many don't need to sell. But many do sell for BTC.