Microsoft is suing an eBay "power seller" with more than 2,000 successful transactions on the online auction site for allegedly selling counterfeit versions of some of Microsoft's best-selling applications and labeling them with fake certificates of authenticity.

In the lawsuit, Microsoft accuses Donald Collins, a Mount Prospect, Ill., resident whose eBay name is ChicagoComputerBroker, of counterfeiting and auctioning off a range of Microsoft applications, including Excel 2003, Outlook 2003, PowerPoint 2003, and Word 2003. Microsoft also accuses Anne Collins, a Chicago resident who operates on eBay as irish5025, of participating in the alleged scheme.

The suit was filed last week in U.S. District Court for the Northern District of Illinois. Microsoft is seeking unspecified damages against both defendants.

Microsoft says its investigators bought software on eBay from ChicagoComputerBroker on several occasions through January. On each occasion, Microsoft "analyzed the software (CD-ROM) and determined it was counterfeit," according to court papers. In some instances, "the product keys accompanying the software were not authorized for use or distribution with that software," Microsoft charges.

Microsoft claims Collins also distributed fake software products at computer trade shows and through e-mail solicitations.

Microsoft further alleges that its investigators found bootleg software for sale on eBay by irish5025 on nine occasions between April and June 2006. Microsoft says the defendants used the proceeds from the auctions and other allegedly illicit sales to purchase real estate and motor vehicles.

Microsoft says it asked eBay to remove the offending items from its site and that the online auctioneer complied.

It's not the first time eBay has been caught up in a dispute over allegedly fake merchandise. In 2004, luxury jeweler Tiffany sued the auction site, claiming that 73% of the so-called Tiffany products listed for sale on eBay are fakes.

Despite the lawsuit, ChicagoComputerBroker continues to list items for sale on eBay. As of Tuesday, he was offering Symantec's Norton Antivirus software for sale but no Microsoft products. ChicagoComputerBroker maintains a positive feedback rating on eBay of 99.3% on more than 2,000 transactions. Depending on how the lawsuit turns out, that could change.

Donald Collins didn't respond to e-mail requests for an interview. The relationship between him and Anne Collins wasn't immediately clear.