By Deepa Seetharaman SAN FRANCISCO (Reuters) – EBay Inc will begin running advertisements on its mobile app in the fourth quarter, moving the e-commerce company into the fast-growing and lucrative market for ads on devices such as smart phones and tablets. EBays users spend 150 minutes on its app per month, three times higher than its nearest rival, said Stephen Howard-Sarin, eBays head of display advertising in North America. Berkshire Hathaways insurance unit Geico [BRKGE.UL] is among the first advertisers on eBays mobile ad network. Advertising is a much higher margin business than retail, but the move risks drawing people away from eBays site.