YOU buy a television on eBay. When it arrives, you eagerly unwrap it, only to find it is badly scratched. You return it, and leave a negative comment about the seller on the site. The next day, you find the seller has retaliated by posting a nasty comment about you, branding you as a time-waster. Suddenly, no one wants to sell to you and your reputation is in tatters.

Until now eBay's rating system, which allows users of the auction and trading site to leave good or bad comments about their trading partners, has worked well. Sellers who ship out damaged goods, or items that do not match their online description, rightly get a black mark against their name. However, this system has recently come under increasing pressure from an all-too-human failing: spite. Sellers can easily retaliate against buyers who have named and shamed them, leaving unwarranted but highly visible ...