Amazon posted a note Friday that the maintenance was continuing and did not say when it would be complete.
"We're very sorry that this has reduced functionality," spokeswoman Carrie Peters said, adding, "We don't have a timeline on (fixing) this."
Sellers say items usually listed in both zShops and Marketplace are no longer showing up in zShops, meaning buyers searching the storefronts area or browsing in individual zShops aren't necessarily seeing the full inventory of products.
"My sales have gone away completely," said Joe, a bookseller in Boca Raton, Fla., who declined to give his last name. Noting that Amazon makes a commission on sales placed through zShops and Marketplace, he added, "there's a lot of money they're not making."
In addition to the display problem, zShops sellers have been unable to edit, re-list or delete their storefront listings directly through Amazon's site. The company has offered a workaround, which entails sellers uploading and downloading their inventory as a spreadsheet file, but some sellers say that's not much of an option.
Donna, Joe's wife, who has her own zShops storefront, said she doesn't maintain a list of her inventory offline. She normally updates her online inventory list at Amazon on a regular basis and is upset that Amazon is asking her to change her methods.
"I don't work like that," she said. "We didn't sign up to do a workaround for 10 days."
Although best known for being the largest online department store, Amazon has been increasingly turning to third-party sellers to diversify the goods and services offered through its site. Over the last several years, the company has teamed with big retailers such as Target and Circuit City, as well as set up areas on its site where smaller sellers can sell used goods via auctions or through the company's new product listings.
Such arrangements tend to offer high-margin revenue, an increasingly important consideration for a company that has been under constant pressure to post profits. But the deals have been a source of trouble for Amazon in recent months.
Toys "R" Us, one of Amazon's first high-profile partners, is trying toits deal with Amazon. Travel sites Expedia and Hotwire have that they haven't seen the returns they expected from their partnerships with the e-commerce giant. Overstock.com selling used computers with Amazon last fall after seeing poor sales.