I imagine these come under eBay's policy on the sale of medical devices and equipment: https://www.ebay.co.uk/help/policies/prohibited-restricted-items/medical-devices-equipment-policy?id...
I'm not able to assist on whether your devices or others that may be on sale here comply with this policy.
The sale of defibrillators doesn't seem to be restricted to registered business sellers only. However, it seems an unusual item to sell as a private seller. Remember that you're limited to selling your own, unwanted possessions. You couldn't sell defibrillators commercially without upgrading to a business seller.
