NEW YORK—Some people who sell things on eBay are fed up with new rules the company has been imposing in hopes of making the auction site more attractive to online shoppers. Now even more changes are coming in the next few weeks, but this time eBay Inc. hopes it can cool tempers.

Already this year, eBay has tinkered with its fee structure, search results and feedback system. These efforts might be meeting eBay’s aims of improving the experience for buyers, but several sellers say their relationship with eBay is worse than ever, and some have left the site entirely.

Jonathan Garriss, executive director of the Professional eBay Sellers Alliance and head of Gotham City Online, which sells shoes on eBay, said his group’s members are seeing fewer of their listed items sell, and lower average prices for things that do sell.

EBay has been rejigging its vast Internet marketplace in hopes of turning around a troubling trend: Its number of active users is barely rising. In the most recent quarter, the figure rose 1.4 percent to 84.5 million.

One big change came in January, when eBay altered its complex fee structure and said it was trying to encourage sellers to offer more items for sale, which in turn could attract more buyers.

Generally, eBay cut the fees it charges for listing an item, but raised its commissions on completed sales of products auctioned for less than $1,000 or sold at fixed prices lower than $100. Meanwhile, the company began taking a lesser bite out of higher-end fixed-price sales—as much as 4 percent instead of a previous maximum of 5 percent.

At the time, eBay said more than 60 percent of its sellers would save money under the new rules. But plenty of complaints poured in. EBay responded by cutting listing fees by as much as half for items in its “media” category that sell for under $25.

Still, many sellers were still unhappy that unlike in the past—when eBay consistently talked of a level playing field for brand-name companies and weekend attic-raiders alike—a new top tier of vendors seems to have an easier time flooding the marketplace. Under a new “Diamond PowerSeller” plan, the highest-volume merchants may be eligible for reduced fees.

EBay says auctions are not going away, but fixed-price sales are the fastest-growing part of the company’s marketplace, increasing 60 percent a year.

For some sellers, like Michael Knight, who dismantles motorcycles and sells the parts on eBay from Garland, Texas, the sheer volume of recent adjustments has been frustrating.

Knight would like to move off eBay, but says it’s difficult to transfer his listings to another site. Other sites will not easily accept the photos embedded in his item descriptions, and modifying every one of his almost 4,000 listings “is just not practical.”

Bruce Hershenson of West Plains, Miss., had spent 10 years selling vintage movie posters on eBay. Instead he now does that twice a week on his own site, eMoviePoster.com, using technology offered by AuctionAnything.com Inc.

Even with a discount he could get by keeping his customer-feedback ratings high, he expected to pay eBay almost $20,000 more per year.

Sellers have also bristled at changes in eBay’s feedback policy, one of the site’s traditional hallmarks. In the spring, the company removed sellers’ ability to leave negative or neutral feedback for buyers, though buyers can still offer negative assessments of sellers. EBay also adjusted its search engine so that items being hawked by people with poorer feedback ratings come up lower in search results.

Some sellers complain that this put them at the mercy of unscrupulous buyers who try to take advantage of the rating system.

EBay’s Norrington said that sellers can report such abuse, and that the company hasn’t seen it much.

Even with the rancor, some sellers clearly have benefited from eBay’s changes. Steven Holt and his wife, Crystal, who sell DVDs from Denison, Iowa, say they’ve seen record sales since the spring, when eBay search results began favoring vendors who, like them, have high feedback ratings.