If you buy anything the payment will come from your balance.   Also even though you have a private selling account as you're selling new goods it should be a business account and it's possible that ebay are charging you accordingly.   You can check here:
https://www.ebay.co.uk/sh/fin/transactions

If necessary you can contact ebay about it by requesting a callback on here:
https://www.ebay.co.uk/help/eua?id=5190