If using managed payments, you should have authourised ebay to debit your bank account for refunds if there is no money in your managed payments account. This should be ideally the same account that money is sent to.
If you have money paid to you on a daily basis, then there will never be money available for this sort of thing, so you have to give ebay a method of payment.
I thought that this was automatic with managed payments, as when a refund is required looks bad to the buyer if you cannot refund.
https://www.ebay.co.uk/help/selling/managing-returns-refunds/refunding-buyers?id=5182&st=3&pos=1&que...
https://www.ebay.co.uk/help/selling/fees-credits-invoices/setting-changing-payment-method-fees-selli...
