I help my friends and neighbors for free anytime they ask me. They return the favor also.
That being said, if I was going to do it for hire, I would NOT give my time away. Also if I was going to do it for hire, and advertise, put up signs, etc, I would make sure I was fully legit, insurance, license, etc. All it takes is one accident, or "unhappy" customer to report you and it all comes to a stop, fines, etc.
I did some bush hogging for realtors for awhile, but it was always last minute type jobs that I really didn't care to do, and it seemed that they always wanted more then they where willing to pay for. Then it was weeks until I would get paid because it was all done through the realtors trying to sell the property. The pay was good, but you can only work so many hours in a day, and I decided that if I wanted to work 12 hour days, I would be better off putting them into my day to day job.