(Updates numbers from Adobe and RetailNext)

By Nandita Bose and Nathan Layne

CHICAGO, Nov 30 (Reuters) - U.S. shoppers seeking to avoid the holiday crowds at bricks-and-mortar stores and snare deals on Cyber Monday came face-to-face with traffic jams and product sellouts, with some customers of Target Corp forced to wait in a virtual line.

Sales on Cyber Monday, traditionally the busiest day of the year for Internet shopping, were expected to finish up 12 percent from a year earlier at $2.98 billion, according to Adobe Digital Index, part of Adobe Systems Inc, which provides digital marketing and media services to merchants. As of 10 a.m. ET on Monday, sales registered at $490 million.

But 13 out of 100 product views returned an out-of-stock message Monday morning, more than twice the rate on an average day, according to Adobe.

"Consumers are hyped for Cyber Monday, with social buzz more positive than what we saw on Black Friday, but they need to brace themselves for the highest out-of-stock rates of the season so far," said Tamara Gaffney, principal analyst at Adobe Digital Index.

Website outages and slow checkouts during the five-day shopping spree that started on Thanksgiving were reported at luxury retailer Neiman Marcus, Wal-Mart Stores Inc, L Brands Inc's Victoria Secret and Foot Locker Inc. Slow service was also reported at payments processor PayPal Inc .

The data underscores the ongoing shift in shopping online, which makes up for slowing spending in stores. But product shortages raise broader concerns about inventory planning by retailers during the most crucial time of the year.

Adobe tracked 80 percent of all online transactions from the top 100 U.S. retailers and said sales are on track to meet its expectation of a record $3 billion by the end of Monday.

Retail stocks including Amazon, Wal-Mart, Target and Macy's Inc fell between 1 and 3 percent on Monday.

Data from RetailNext showed overall in-store sales from Thursday to Sunday fell 4.7 percent, with customer traffic down 5.1 percent .

Data from ChannelAdvisor, a retail technology and services firm, showed marketplace sales at Amazon, which account for more than 40 percent of Amazon's total revenue, grew 22.4 percent, Google Inc's Google Shopping marketplace grew 14.34 percent while eBay Inc rose 1.9 percent from midnight to noon Eastern time on Monday.

On Target's website, www.target.com, some shoppers looking for bargains were greeted with an error message: "So sorry, but high traffic's causing delays. If you wouldn't mind holding, we'll refresh automatically & get things going ASAP."