[BjoernKW]

Freelancing always is a business. It doesn't matter if it's run on the side.

While you can outsource aspects such as marketing and sales you should be aware that in that case not only will you be leaving a lot of money on the table (because someone else higher up the value chain will capture that value for you) but you'll be dependent on third parties other than your clients, which means that your freelancing income can dry up at any time.