This simply saves time (Score 3, Informative)

This is eBay cooperating with law enforcement because it saves paperwork and time. Handing the information over to law enforcement is inevitable, because most WILL go through the process of having a subpoena or warrant issued, especially if the information is critical to the case.

Prior to that, a phone call is all that is necessary to a service provider to legally obligate them to preserve whatever records they already have for the given subject. This power comes from 18 USC 2703 (f) and is known as an "Order to Preserve." It does not require the service provider to start collecting new information, or collect more than they previously were, just to preserve what they already have. That gives law enforcement time to draft the court order and get it signed.

Legal counsel at service providers know these issues very well. eBay is apparently choosing to make life easier on the legal end of things by offering a certain level of cooperation. Notice it did say they would require a warrant in certain situations, so it's not 100%.