See this report. Is this a form of illegal vote buying? The issue is currently being litigated in California. Relying in part on my article, Vote Buying 88 California Law Review 1323 (2000), the ACLU is arguing before the district court that the reasons we generally think about for making vote buying illegal—that it is inegalitiarian, inefficient, or violates an inalienability norm—do not apply to vote trading between voters in different states. The ACLU further argues that vote trading on these websites is unenforceable, and therefore speech activity protected by the First Amendment under Brown v. Hartledge. Professor Jamie Raskin, quoted in the linked article, has been advocating such vote trading since 2000.