Carl Icahn sunk his teeth deeper into eBay (EBAY) Monday, claiming to have evidence showing CEO John Donahoe’s “incompetence” cost shareholders $4 billion.

This comes as the eBay board rejected Icahn’s two board nominees, employees Daniel Ninivaggi and Jonathan Christodoro, saying both were unqualified and urging shareholders to vote against them at its upcoming annual meeting.

eBay founder and chairman, Pierre Omidyar, urged company stockholders to support the company’s nominees, a list that includes Donahoe.

Icahn, the billionaire investor who upped his stake recently in eBay, says he has uncovered “indisputable” evidence related to Donahoe’s handling of eBay’s 2009 Skype deal, which has been of much debate between the two parties in this escalating spat.

The online marketplace sold the Internet telephone and video service to an investor group that included director Marc Andreessen’s venture capital firm in a move that valued it at $2.75 billion. Just years later, Skype was sold to Microsoft (MSFT) for $8.5 billion.

“We believe based on evidence we have newly uncovered that Donahoe’s inexcusable incompetence cost eBay stockholders over $4 billion,” Icahn said.

He says Donahoe oversaw a “premature” sale of Skype at a diminished price by selling a controlling interest to Andreessen rather than resolving outstanding litigation between eBay and Skype’s founders, which apparently caused Microsoft to initially walk away from the deal in 2009.

Andreessen, Icahn claims, then resolved the outstanding licensing litigation and flipped Skype to Microsoft for nearly triple the price -- a roughly $4 billion spread Icahn claims should have been paid to eBay’s shareholders.

“The obvious unanswered question is: why didn’t Donahoe, who allegedly also had the information that would be key to reaching a settlement with Skype’s founders, settle it for eBay and sell Skype directly to Microsoft, thereby keeping the extra $4 billion for eBay stockholders?” Icahn said.

The investor once again urged eBay shareholders to reconsider Donahoe as CEO, or “at the very least” send a message that they are “very unhappy” with the current situation at eBay. He also continued to call for an immediate PayPal spinoff, saying he believes there would be a “number of bidders” willing to pay a large premium for an independent PayPal.

In response to Icahn’s criticisms, Donahoe said eBay got the “highest value at the time” for Skype and that Andreessen recused himself throughout the Skype process despite Icahn’s criticisms that there were conflicts of interest that led to a mishandling of the deal.

On Friday, Andreessen issued a public letter calling out Icahn for his own conflicts of interests related to his unrelated investment in XO and purchase of the telecom giant’s wired business.

“Carl Icahn 2014 would take a dim view of the business ethics of Carl Icahn 2005,” Andreessen wrote in the letter.

Shares of eBay fell more than 1.5% to $58.14 in recent trade. They are up about 9.5% over the last 12 months.

Follow Jennifer Booton on Twitter at @Jbooton