Online marketer EBay is buying GSI Commerce, a Montgomery County company that runs websites, ships products and provides various marketing services. It has long-term contracts with 180 retailers including Toys R Us, Bath and Body Works, Ace Hardware and Radio Shack. EBay is paying $2.4 billion for GSI, which is based in King of Prussia. E-Bay Chief Executive Officer John Donahoe says commerce has reached a point where online and offline selling are blurring, and he is looking for retailers of all sizes to help them grow their businesses. It might also help the company compete with Amazon, which sells a wide variety of merchandise online.