It is illegal to sell alcohol without a licence. In order to sell beer at a Saturday market, you would need a full off sales licence, transferred there for the day.
If the individual in question brewed the beer in his or her home, he is unlikely to have a brewers licence, which means he has no business selling beer at all, even in wholesale quantities to a licence holder. This would also mean that he or she has not paid any duty on the alcohol.
I would not like to be in his or her shoes when the nice people from Customs and Excise catch up with them.