By Alistair Barr

SAN FRANCISCO (Reuters) - The performance of eBay Inc's main Marketplaces business will dominate results from the e-commerce company later this week.

EBay has been working on a turnaround of its online marketplaces for over a year and growth has begun to pick up in recent quarters.

EBay shares are up more than 20 percent so far this year, but further gains will depend on growth continuing, or even increasing, analysts and investors said.

"Continued acceleration at the core eBay Marketplaces will be of paramount importance," said Fred Moran, an analyst at Benchmark Capital.

EBay's Internet marketplaces, the biggest of their kind, bring together buyers and sellers and the company makes money by charging fees on transactions and other activity.

The marketplaces business started as an auction website, but eBay has moved away from that model and now about 60 percent of sales are fixed price.

Having more fixed prices has made eBay's marketplaces more accessible to a broader audience of shoppers, helping the company compete better with Amazon.com Inc, the world's largest Internet retailer.

"The transition to fixed-price has paid off and enhanced the growth rate," Moran said. "If that continues in the third quarter, the stock should be rewarded.

EBay is expected to report earnings of 48 cents per share on revenue of $2.91 billion, according to Thomson Reuters I/B/E/S. The company is due to unveil results after the stock market closes on Wednesday.

When eBay reported second-quarter results in July, the company forecast third-quarter revenue between $2.85 billion and $2.95 billion and earnings of 46 cents to 47 a share.

EBay also forecast full-year revenue of $11.3 billion to $11.6 billion in July and profit of $1.97 to $2.00 a share.

A closely watched metric will be the Gross Merchandise Volume, or GMV, that changed hands on eBay's marketplaces, excluding vehicle sales.

"How people react to the earnings will depend on how much GMV has been transacted," said Bill Smead of Smead Capital, which owns eBay shares. "We expect not spectacular but steady improvement there. EBay is participating in the growth of e-commerce, while a few years ago they weren't."

Justin Post, an analyst at Bank of America Merrill Lynch, is expecting U.S. GMV growth of 14 percent in the third quarter, versus a year earlier. But growth could be as high as 16 percent, he noted.

The focus on fixed price sales makes eBay a much more effective destination for holiday shopping, while the company has also reduced clutter on its website and improved cataloging and search functions, according to Post.

(Reporting by Alistair Barr; editing by Andre Grenon)