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.
From Ernest Miller of LawMeme, via Patrick Nielsen Hayden, both of whom have some thoughts on the subject.