That's not how copyright law works. The user owns the copyright, and merely grants Craiglist a license to it. Note that a legal argument could have been made

That's not how copyright law works. The user owns the copyright, and merely grants Craiglist a license to it. Note that a legal argument could have been made that based on the new (but since abandoned) Craiglist TOS, by posting to Craiglist a user was revoking all prior grants of license to other sites, i.e., including any licenses granted to BidKat (by virtue of using BidKat). In such case, you would have been the one in CL's crosshairs, not the user.