Once you accept an offer, the buyer is notified to complete payment. Buyers have up to 48 hours from acceptance to make payment.
Most buyers pay promptly. If the buyer hasn't made payment within 48 hours, the deal is automatically cancelled and your advert is re-listed so other buyers can make an offer. You don't need to do anything.
Once the buyer has paid, you'll be notified as soon as the payment clears and it's safe to ship. An EFT can take a day or two to clear, so a payment made before the deadline still counts even if it clears a little later.