SAN FRANCISCO – Computer software makers launched a crackdown on illegal Internet sales of their products yesterday by suing suspected pirates who have set up shop on the popular online auction site eBay Inc.

Usually fierce rivals Symantec Corp. and McAfee Inc. teamed up to kick off the crusade by targeting five different eBay sellers in three lawsuits filed Monday in a Los Angeles federal court.

“If online marketplaces are going to pursue the free-market ideals that they aspire to, they must make sure the products they sell are authentic,” said Joe Fitzgerald, Symantec’s vice president of intellectual property.

The two leading makers of antivirus software decided to sue after uncovering evidence that the individuals named in the complaints had completed more than 15,000 sales involving pirated software between October 2005 and December 2005, said Keith Kupferschmid, an executive with the Software ‘ Information Industry Association.

The trade group is coordinating the software industry’s efforts to patrol eBay and other Internet auction sites for pirates. Kupferschmid said the group intends to buy copies of pirated software in the auctions and then sue “egregious” copyright violators without forewarning. The industry expects to file the suits on a monthly basis.

The campaign isn’t currently aimed at eBay or the buyers of pirated software.

Besides software makers, the association also represents a large number of providers of electronic information, including The Associated Press.

This week’s initial burst of lawsuits named: Edward Cosmos of Bloomington, Calif.; Grace Chan of San Jose, Calif.; Kevin Liu of New Brunswick, N.J..; Mary Tian of New Brunswick, N.J.; and G.T. Tian of Highland Park, N.J.

Cosmos, Liu and the Tians didn’t immediately respond to e-mails sent to their eBay profiles yesterday.