EBay is happy to help, officer:
EBay has probably the most generous policy of any internet company when it comes to sharing information. […]
Our policy is that if you are law enforcement agency you can fax us on your letterhead to request information: who is that beyond the seller ID, who is beyond this user ID. We give you their name, their address, their e-mail address, and we can give you their sales history without a subpoena.
We also do other things to facilitate your [law enforcement] investigation by looking around and doing some searches on our own, typically to see if there are some other user IDs associated with that thing.