Craigslist Inc. co-founder Craig Newmark said he would have canceled the sale of a stake to eBay Inc. had he known the auction company would use confidential data to start a competing classified-advertising Web site.

Advance warning that eBay wanted to use Craigslist data to help set up the Kijiji site would have been “a deal breaker,” Newmark testified in a trial over eBay’s claims that Craigslist unfairly devalued eBay’s 28 percent stake.

EBay is asking Delaware Chancery Court Judge William B. Chandler III to find Craigslist officials violated Delaware corporate law by issuing shares to themselves to dilute eBay’s stake and caused as much as $110 million in losses tied to the investment.

EBay, based in San Jose, Calif., contends Newmark and Craigslist Chief Executive Officer Jim Buckmaster unfairly stripped it of a Craigslist’s board seat that accompanied the investment.