Vendors and shoppers are complaining that Google's Checkout often takes too long to complete sales transactions and sometimes cancels orders unjustifiably and without warning.

The problem apparently stems from the review process Google performs on the Checkout transactions to prevent fraud. While they applaud antifraud efforts, users of this high-profile service, which was launched in late June, say Google needs to speed up the review process and improve its review-related customer service and communications. Otherwise, Checkout, which observers have characterised as a potential 'killer' of rival service PayPal, risks failure, some warn.

Google hasn't replied to a request for comment, but at its website, the firm explains that it employs standard credit-card verification methods in Checkout, as well as more specialised risk modelling, fraud detection and manual reviews if deemed necessary.

Moreover, in the official Google Checkout Merchants Forum, a Google staffer in the Checkout team routinely posts comments and answers under the name GoogleCheckoutPro, and on 27 July addressed the review delay issue, saying Google is committed to fighting fraud and minimising risk.

"I know there's frustration about the delays due to the order review process," the official wrote, adding to what is one of the longest threads on the board with over 30 postings.

"At times it may seem we are overly cautious, especially when an order from someone you know personally is being held up. We are working as fast as possible to fix this issue. The good news is that we have some upcoming changes that will both speed up the review process and make it more effective at filtering out the bad guys."