Five months ago, Klaus Priebe, a soft-spoken building contractor who said he was sick and tired of fraud on eBay, decided it was time to catch the cheaters at their game.

In one recent auction, he bid as much as $2.5 million on a telescope worth no more than $2,000. He knew he would not have to pay for the telescope because he was sure that it did not exist.

The listing was a fake, he decided, because the seller offered free shipping and was registered in Andorra, a small country in the Pyrenees that is often listed by swindlers. Mr. Priebe said his wild bid was an attempt to protect innocent bidders from falling into the trap he had spotted.

Mr. Priebe, 42, is an eBay vigilante, one of a number of eBay members who are stepping in to fight online auction fraud — a problem they say is getting worse by the week — because they believe that the company does not do enough policing of its own.

But in eBay’s view Mr. Priebe and his vigilante brethren are pariahs. Rather than embrace these virtual posses, eBay discourages them, occasionally going so far as to suspend the vigilantes’ accounts.

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