A powerful and vocal group of large eBay merchants has blasted the e-commerce giant for changes it has implemented this year, saying they have done more harm than good.

The Professional eBay Sellers Alliance (PESA) on Tuesday published a scathing indictment of eBay, charging that most merchants have been hurt by recent modifications to the online marketplace.

Changes to the way buyers rate sellers, to eBay's fee structure for listings and to the search ranking functionality have been flawed and have led to a substantial deterioration of the marketplace for merchants, according to PESA.

In an interview Thursday, PESA Executive Director Jonathan Garriss reiterated the complaints. "The sentiment of most eBay sellers is that 2008 has been a very difficult year, and they're really nervous because they're not seeing any of the changes eBay is implementing improve their business," he said.

eBay didn't immediately respond to a request for comment.

Although PESA represents merchants with very high sales volumes -- its roughly 700 members collectively sell more than US$400 million in gross merchandise volume per year on eBay -- Garriss maintains that the eBay changes in question are affecting all types of sellers.

In the case of PESA members and many other so-called Power Seller merchants who make their livings selling on eBay, the situation has serious consequences, said Garriss, who is CEO of eBay merchant Gotham City Online, a seller of shoes.

To earn a Power Seller badge from eBay, merchants have to maintain certain customer satisfaction levels and sell a monthly average of between $1,000 or 100 items (Bronze level) and $150,000 or 15,000 items (Titanium level). Power Seller status grants merchants certain benefits at eBay. PESA members are mostly Platinum and Titanium Power Sellers, the top two categories.

"Their livelihood is being threatened because the marketplace isn't doing as well as a lot of Power Sellers think it could," Garriss said.

For example, eBay had the right idea when it decided to use buyer ratings to increase or decrease merchants' visibility in eBay's search results and to award listing fee reductions, he said. However, the plan's implementation leaves a lot to be desired, he said.

In addition to the general positive, negative and neutral ratings for the overall transaction experience, buyers can also leave what eBay calls Detailed Seller Ratings (DSRs) in four specific areas, with a scale of one to five stars: accuracy of item description, communication, shipping time, and shipping and handling charges. The search visibility and fee rebates are based on these DSRs.