You could even look at it as an extension of homebrewing but with the chance to make a little money, which is more than I make homebrewing
Just be aware that you will most likely need to brew on at least a 3 bbl scale to make any money to compensate your time. Also, as Thirsty has mentioned - it may not be legal to brew on your premises (but it might depending where you live.)
When I was homebrewing on my 12 gallon system I was downright greedy with my beer. I never gave away growlers and I would not have entertained selling either even if it was legal simply because no one would have wanted to pay what it was worth to my time. If you came over to my house I would gladly drink the beer with you, because that was part of the fun. But 12 gallons, let alone 5 gallons, of homebrew is some of the most expensive beer in the world if you look at the time involved.
That is why I never had your beer! I have to go to your house. Makes sense now. LOL!