They are presumably offering your business a concession that requires you to provide your service at a market site on a Sunday. How you achieve that is up to you - they are not employing you.
It appears that you are responsible to cover your 'pitch', as a self employed person, there is nothing stopping you from employing someone to cover the pitch to give you a day or days off
Your choice - pay for the pitch and do some business or lose the pitch and pay for the rent of a shop. YOur sel;f employed, you don't "have" to do anything - this is a business decision
As a market trader you are treated with kid gloves compared to shopkeepers who, through Business Rates, effectivly pay for the pitch 365 days of the year