Call it the rebuttal to the rebuttal. Carl Icahn on Wednesday fired off his second open letter to eBay shareholders this week, reiterating a call for the ouster of board members Marc Andreessen and Scott Cook and once again demanding the company spin off its PayPal unit. Apparently he was unmoved by CEO John Donahoe’s e-commerce tutorial outlining exactly why the spin-off isn’t in the cards nor the company’s immediate and dismissive response to Monday’s missive.