I find for the most part buyers are honest. As a seller I wouldn't get to worried about it just yet.

For instance the customer may have gone through the checkout and written down the bank details and waited until tuesday or wednesday to go to the bank and pay. They could have run out of money and needed a little more time to pay, which is slightly embarrassing, hense no response to emails.

You could check and confirm all their details in the meantime to give some reassurance.

Cheers, Dylan