You, like many others before you, are confusing the seller account type (the legal entity), and the selling plan. The Individual Seller Account is ONLY for selling off your own personal second-hand items (eg. books, DVDs, CDs etc) and cannot be used to sell NEW goods.

The professional selling PLAN is where you pay £30 a month, and as soon as you click on the orange 'sign up' button on the Amazon page, they automatically sign you up for the professional selling PLAN.

You can downgrade the plan until you are able to start selling once your account is fully verified. Go to settings (cog wheel top right) - account info - sell on Amazon - same again - and tick the individual button instead of professional. If you have not sold anything you can claim a refund of the unused fee by opening a case with seller support - Help.