code: public CoinbaseTransfer buy(BigDecimal quantity) throws IOException {
  
      final CoinbaseTransfer buyTransfer =
          coinbase.buy(
              quantity.toPlainString(),
              false,
              exchange.getExchangeSpecification().getApiKey(),
              signatureCreator,
              exchange.getNonceFactory());
      return handleResponse(buyTransfer);
    }
docs: Authenticated resource that lets you purchase Bitcoin using the primary bank account that is
  linked to your account. (You must link and verify your bank account through the website before
  this API call will work). The underlying optional parameter agree_btc_amount_varies is set to
  false. Use {@link #buyAndAgreeBTCAmountVaries} to have it set to true.
  
  @param quantity The quantity of Bitcoin you would like to buy.
  @return The {@code CoinbaseTransfer} representing the buy.
  @throws IOException
  @see <a
  href="https://coinbase.com/api/doc/1.0/buys/create.html">coinbase.com/api/doc/1.0/buys/create.html</a>