Well,
Joel Salatin would say to sell your birds LIVE (as someone else eluded to prior) and then offer free butchering!!! to your clients. On a small scale, you will have a case (may not win) in court that you just sold them a live chicken. They followed up by asking you to process for them as a favor.
If you are butchering other folks birds, you will need to dive in further. General liability ins. is always recommended and does not cost much. However, I would assume that the insurance agent writing the file will want to see good history with you as a worker of a butcher shop/experience/and possibly a USDA inspection id etc-- shop around, you may find an insurance company with easier qualifications and just want your yearly fee. Most biz insurance for $1mil umbrella (low coverage) is $500 per year depending on line of work. I have not received a quote for livestock/butchering/meat sales though.