A seller who said they weren't a business but an individual selling their own used items can choose one of two selling plans. One is 75p extra per sale, the other is £30 per month but no extra 75p per sale.

The former is called the individual selling plan, the latter the professional selling plan. You chose the professional selling plan.

If that's not you want then go into your account and choose to downgrade it. You'd then have to open a case and ask Support for a refund.