An increase in buyer satisfaction ultimately benefits sellers. "Serving relevant, compelling ads in certain situations makes for a better buying experience," Durzy said.

Still, eBay is considering reactivating the in-house ad program for eBay sellers that it cancelled last year, because it recognizes that it had some value. "We're examining parts of that [program] that we can reconstitute within the context of our deal with Yahoo," he said.

What's not negotiable is the existence of external ads on eBay. "We continually test what's the best mix and how to improve our targeting so that we serve the right ads to the right people and avoid that thing which sellers legitimately fear," Durzy said.

Recently, Steve Hartman, eBay's director of on-site advertising strategy, addressed these issues in a blog posting. "We want buyers to transact on eBay," he wrote. "But we also want to offer alternatives when we believe it will improve the buying experience."

eBay won't claim that the ads have never driven a potential buyer away from the marketplace, but those situations would be exceptions, not the rule, Durzy said.

"Our initial testing period shows no significant impact to our core transaction business, which means no significant impact to our sellers' revenue, because the two are inextricably linked," Durzy said. "We feel very confident that our strategy is working."

Garriss, also CEO of Gotham City Online, an apparel store on eBay that also has its own site, isn't convinced.

"We think it hurts the sellers, while eBay benefits from the ad money," he said. "This is one of our fastest-growing concerns."