I Have A Coinbase Account
I Sell Bitcoin $20.00 at Market

ORDER DETAILS
MARKET
AMOUNT                   0.00078578 BTC
TOTAL                   20 USD
EST FEE                   $0.12
EST SLIPPAGE            0.00%
EST AVERAGE PRICE    25605.44 USD

[SELL BTC]

I click [SELL BTC] and the order is placed.
The order is immediately filled and over.
I made $20

ORDER DETAILS
ORDER ID
PAIR                   BTC-USD
TYPE                   Market
TIME IN FORCE   Immediate or Cancel
SIDE                   Sell
AMOUNT           0.00078578 BTC
EXECUTED           0.00078578 BTC
SUBTOTAL           $20.12
FEE                    $0.12