I don't think you can.

Bpay is a bill paying facility. eBay take their fees out at point of sale. Any other fees they will take directly from your bank account, if you don't have enough pending disbursement.

As you are a buyer, I doubt Bpay would be available. As said it's for paying bills, not buying things. Use Paypal, a credit card or any of the myriad other payment options eBay provides. Which Bpay is not one of.