All buyers and sellers are equal in the virtual auction world of eBay, but buyers are about to become more equal than everyone else.

From May, eBay, the world's biggest online auction house, will block sellers from posting any negative or neutral comments about buyers who purchase their goods to coax more of them on to the site.

The move means that anyone selling an item will have little recourse to complain that the purchaser of their Britney Spears T-shirt or antique Whitby egg-timer had been a late payer, or had become a persistent troublemaker, grumbling unfairly about their acquisition.

At the moment, eBay buyers and sellers are encouraged to give a rating about their counterpart on the auction site's feedback system, which alerts other customers about bad experiences and helps to self-regulate the auction site. A buyer or seller with a bad rating is effectively blacklisted. At present customers can scroll through several months of comments about both buyers and sellers to decide whether they feel comfortable doing business with them. Frequent gripes include a seller over-charging for postage, a purchased item delivered late or one party being generally dishonest.

EBay has defended the move, claiming that buyers need more protection than sellers because they have to send money to a counterparty they do not know.

A spokesman for Ebay said yesterday that the auctioneer wanted to stamp out an increasing practice where sellers who had received a bad rating would retaliate by posting a poor rating on the buyer. "Some sellers are gaming the system. And some buyers in turn have been turned off," the spokesman said.

The group added that it is rare for buyers to renege on payments. But sellers have claimed that the new system leaves them vulnerable to extortion with buyers wielding the ability to blacklist them without the opportunity to defend themselves.

A week ago eBay - which makes the bulk of its revenue from commissions - announced that it cutting the amount it cost to list an item on the site by a third, but increasing the commission it charged on completion from 5.25 to 7.5 per cent.

For the vast majority of sellers, this had the effect of increasing the overall cost of shifting goods on the site. For instance, to sell a £100 camera now costs £6.70, where previously it would have cost £5.85.

The higher commission means that sellers of high value goods now also pay more.

Sellers have claimed that the new system leaves them vulnerable to extortion, with buyers wielding the ability to blacklist them without the opportunity to defend themselves.

A week ago eBay - which makes the bulk of its revenue from commissions - said that it was cutting the amount it cost to list an item for sale on the site by a third but was increasing the commission it charged on completion from 5.25 to 7.5 per cent.

The new pricing structure unleashed a torrent of complaints on eBay's message boards, where its sellers - who have long been a vocal community - vented their anger.

One said: “The fees are deceiving: lowering the front-end cost, then adding it to the final fee. Anyone who can do the math can see that they are not lowering the fees, they are increasing them.” Another said: “As a seller, I have been kicked in the head.”

The latest upheaval comes at a difficult time for eBay. Its core business has struggled against slowing growth rates and it is trying to combat issues such as fraud, as well as increased competition from vendors, such as Amazon.

Last year it said that it was taking a $1.4billion (£712 million) charge in relation to Skype, the telephony service that it bought for $2.6 billion in 2005 and for which it admitted it had overpaid. Analysts have also expressed concern at the failure to increase the number of users, which remains static at 83 million.

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