EBay hired the B-52's rock group for a gala dinner here, sent acrobats up ropes, rented an amusement park and threw open its corporate offices yesterday to thousands of Internet sellers for behind-the-scenes tours as part of its 10th anniversary celebration.

Yet no amount of fanfare could gloss over the rumblings of discontent among the 11,600 people attending eBay's fourth annual user convention. The mostly mom-and-pop sellers who helped build eBay into the world's largest Internet commerce company griped openly about their slowing sales growth and fee hikes that eBay imposed several months ago.

"EBay treated us as disposable sellers for the last couple of years," said Angela Cash of Kennesaw, Ga., who bought a warehouse in 2004 to hold the Internet home decor items she once sold only on eBay. "I have auctions on Overstock.com now and am launching on Yahoo Auctions next month. Amazon has been courting me for two months, too."

The grousing from eBay sellers -- and the fact that eBay rival Overstock.com brazenly set up in a hotel here and lobbied eBay merchants to use its year-old auction service -- reflects the growing pains and intensifying competition throughout the online shopping industry.

Most of Web commerce turns 10 this year, including eBay, Yahoo and Amazon.com. Both eBay and Amazon launched their first services on the Web in 1995, the same year Yahoo incorporated to make a business out of the directory two college students started in their dorm room the year before. Netscape also held its initial public stock offering in 1995, igniting a frenzy among entrepreneurs eager to commercialize what had long been an academic medium.

The next decade produced a boom and bust that left EBay, Yahoo and Amazon, along with younger rival Google, as the Internet's top survivors. All four have been on a tear over the past year as they rushed to copy one another, roll out new services and buy a string of start-ups. Each is positioning itself to catch the next wave of Web commerce. But first they have to figure out what that will be.

"The exciting thing to me is that while we are 10 years into Internet commerce, it is still hard to predict what the next 10 years will bring," said Scot Wingo, chief executive of ChannelAdvisor Corp., a firm that sells automation software to help Internet merchants sell from eBay, Yahoo, Amazon and elsewhere. "After hardly being known a few years ago, Google is on the scene today, changing the rules on a lot of things."

Today's top players thrived by serving the many online retailers that attracted people to the Internet to browse, buy and research goods. In 2004, online retailing accounted for 4.6 percent of total retail sales in the United States, according to data released by the National Retail Federation's Shop.org subsidiary last month. Typing credit card numbers into remote Web stores has become commonplace, with 69 percent of American households now using the Internet to make purchases, Forrester Research reported last month.

Of the big survivors, eBay's success was the most surprising because programmer Pierre Omidyar started it to test of his ideas about pricing goods online and reviving centuries-old notions about community. Folks scoffed at the AuctionWeb program he put on his Web site on Labor Day 1995, offering just three basic functions -- list, view or bid on items. Although it was profitable almost right away, venture capitalists belittled it as an electronic flea market, and even Jeffrey Skoll, the man who partnered with Omidyar to turn it into a business, initially failed to grasp Omidyar's vision.

"I said, 'Pierre, that's a really dumb idea,' " Skoll said at the convention on Friday.

Since then, eBay has exploded into a worldwide economic and cultural force, with about 60 million active users expected to swap more than $40 billion in goods and services this year. While its revenue is less than half of Amazon's, far more merchandise is traded through eBay's person-to-person trading model; it just isn't booked as revenue because eBay doesn't handle merchandise.

Yet eBay's annual revenue growth slowed to 36 percent in the first quarter this year, causing analysts to worry that it eBay could be losing market share. Its annual sales growth in prior years had been averaging closer to 60 percent.

EBay and Google remain the most profitable of the group, but the two companies are regarded differently on Wall Street. At more than $80 billion, Google's market valuation is nearly 80 percent higher than eBay's, more than 50 percent higher than Yahoo's and more than five times Amazon's.

As each company has become recognized as a brand name, it has also come to stand for a particular approach to Internet commerce. EBay is auctions. Amazon is like an electronic mail-order company. Yahoo and Google's search engines link shoppers with retailers.

Sellers who started on eBay have watched the other types of commerce evolve and are finding them increasingly attractive.

Lisa Monse, a collectibles dealer in Houston, said she launched her own Web site two years and has aggressively marketed it by buying keywords on Yahoo and Google. Now she sells nearly twice as much from her personal site, TheVelveteenRabbitAntiques.com, as she does on eBay. "I spend a lot of money on search advertising because it's paying off," she said.

At the convention, eBay directly confronted the reality that its veteran sellers are increasingly selling their wares elsewhere as they grow more Internet-savvy. On Thursday, eBay launched a Web storefront service that lets merchants set up independent Web sites off eBay and pay lower commissions than they would selling on eBay.

"The Internet is changing. It's becoming even more mainstream, and we want to evolve with you however you see fit," eBay chief executive Meg Whitman told nearly 10,000 users packed into an arena.

That need to evolve is causing a flurry of activity among the big Internet commerce competitors, as they cross into each others' territories to add services to woo more consumers.

Amazon not only invites merchants to sell on its site and charges a commission, but it also recently rolled out a trial local Yellow Pages service and a Web search service that people can customize.

Google bought blogging software, photo-organizing software and a satellite-mapping firm -- and moved into shopping by creating a product-comparison service called Froogle. Just this week, Google confirmed it is developing an online payment service that could compete with eBay's PayPal.

Yahoo's original Web shopping area offered links to the sites of established retailers and small merchants, many of whom paid Yahoo to host their sites. But Yahoo has since emphasized a search box as a key way for shoppers to find products, a focus it developed after spending billions of dollars to acquire search technology similar to Google's. Now, Yahoo is the most diversified Internet company, reaping revenue from banner and display ads, search advertising, subscription services for music and e-mail and renting Web storefronts.

For its part, eBay has been on a buying spree to diversify beyond the auction format that accounts for nearly three-quarters of its sales. Last year, it bought e-commerce sites in India, Germany and the Netherlands. In the United States, it bought Rent.com, an apartment-listing site, and took a 25 percent stake in the Craigslist.org local classifieds service. EBay also rolled out a classified network overseas this year called Kijiji. And eBay jumped into search-engine marketing last month, announcing it would fork over $620 million to buy the top comparison-shopping service, Shopping.com.

Search-engine marketing became the wild card of Web commerce after online advertising crashed in 2000. Google and Yahoo pioneered the technique, in which merchants buy keywords such as "diamond ring" or "MP3 player" and have links to their Web sites show up in ads alongside the regular results when people search for those terms.

Merchants pay only when people click on their ads.

Keyword searches quickly became the biggest driver of Internet ad revenue and a source of worry to eBay and Amazon. Both have been big buyers of keywords to attract buyers to their shopping destinations, but individual merchants can use the same approach to bypass eBay and Amazon and reach customers directly.

Moreover, as merchants learned how to use search-engine marketing to bring shoppers to their Web stores, they have been driving up keyword prices at Google and Yahoo -- making it more expensive for eBay and Amazon to acquire traffic that way.

Gary Neubert, a Tampa resident who has sold shipping supplies on eBay since 1999, said he has been steadily building traffic to his own Web site to bypass eBay's commissions whenever possible. Half his sales today come through his own site, he said. Neubert said he and others are increasingly exploring options outside eBay.

"EBay has been a wonderful place to start and incubate a business,'' Neubert said. "Where it has been weak is helping you once you are growing and thriving. The whole Internet commerce evolution has been a fantastic opportunity that only comes along once in a generation, but now we are ready to take it to the next level."