First, it's not a good idea to use your debit card as the secondayr source since any identity theft would expose your entire bank account. I buy and sell a lot on Ebay and applied for a PayPal Credit Card. I sue it and let the proceeds from sales build up in the PayPal account and then pay off the credit card when due.
Secondly, I'm in the U.S. so I don't know what Cex is. But, often (at least here in the U.S.) a hold is placed on any online purchase until a verification from the Debit Card that after the funds were demanded by the seller and were actually transfered by your bank. In fact, many businesses where the end total bill isn't sertain (such as rental cars or online reservations at hotels) won't use a debit card for the guarantee. Rather, they want a credit card for the guarantee but will allow you to pay the final bill with your debit card in person...