code: public CoinbaseTransfer sell(BigDecimal quantity) throws IOException {
  
      final CoinbaseTransfer sellTransfer =
          coinbase.sell(
              quantity.toPlainString(),
              exchange.getExchangeSpecification().getApiKey(),
              signatureCreator,
              exchange.getNonceFactory());
      return handleResponse(sellTransfer);
    }
docs: Authenticated resource that lets you convert Bitcoin in your account to USD by crediting your
  primary bank account on Coinbase. (You must link and verify your bank account through the
  website before this API call will work).
  
  @param quantity The quantity of Bitcoin you would like to sell.
  @return A {@code CoinbaseTransfer} representing the sell.
  @throws IOException
  @see <a
  href="https://coinbase.com/api/doc/1.0/sells/create.html">coinbase.com/api/doc/1.0/sells/create.html</a>