The programme allows users to set up and maintain online stores and process payments.
But problems began at 6am US Pacific time when Yahoo said that systems which power the service went down.
As a result, merchants were unable to process orders and users were unable to make purchases.
Yahoo had restored the service by 1pm, but transactions remained slow to process. The service was not up to normal speed until 12 hours after the outage.
Rich Riley, senior vice president of Yahoo's online channel division, apologised to merchants hit by the outage in a blog posting.
"We deeply regret the inconvenience this caused to our merchants and their shoppers," he wrote. "Our customers' expectations were not met, nor were our own."
But Riley's apology was not enough for retailers that lost business during the busiest online shopping day ever recorded.
"Just telling us the time-line of what happened is not very useful," said customer Wayne Eskridge.
"If you want to gain back the confidence of your customer base you ought to be more specific about what happened, why it happened, and what you are doing to assure us that it will not happen again."
Craig Clark, owner of bedding retailer Pacific Pillows, suggested that Yahoo should take steps to reimburse retailers for the downtime.
"This outage cost us big time in terms of money, time and customer goodwill, " he said. "I think a refund of all fees that Yahoo collected from each store damaged by this outage for the prior six months would be appropriate."
Research firm comScore estimated that customers spent $733m online on Monday, the first time in a single day that online retail broke the $700m barrier.
Retail traffic is expected to get even heavier before the end of the year, possibly logging more than $800m in daily sales.
Yahoo apologises for Black Monday fiasco
By Shaun Nichols on Nov 30, 2007 12:25PM