There can be a commission at the end or there can be an extra % to be made. When I was buying and selling in BTC if I needed to buy or sell BTC I would trade on localbitcoins, buying at 5-10% under the market rate and selling to the market 5-10% above market rate.

Further, while the transfer via the bank is usually a flat fee the bank also make a cut on the exchange rate that they charge the user, so there is a % element to consider on larger transfers.