... and those buyers will lose their accounts with the site, right? I thought that was the whole point of them having to "verify" everyone.
Is it possible to cancel a transaction?
Yes, it is, however you should not do this unless you have a VERY good reason (e.g. your bank is unable to send the payment to the seller's bank). Also, you cannot cancel the transaction by yourself, but you can request the other party to agree to cancel it. To send a cancellation request to the other party go to your Transactions and use the "Cancel transaction" button for the relevant transaction.
You will be asked to type in the reason for cancellation, that will be then emailed to the other party of the transaction. If they deem your reason fair, they will cancel the transaction. Otherwise the only way to cancel it is to email BitMarket. Please remember: buying coins is not a game, these are real people you deal with and by putting an offer out there you give your word that you will fulfill it. If you abuse the Cancel option your account will more than likely be suspended and you may be banned from the exchange.