Buyer-seller watermarking protocols incorporate digital watermarking with cryptography, in order to protect digital copyrights and privacy rights for the seller and the buyer before, during, and after purchase activities in e-commerce. In this paper, we analyze the security of some previously proposed protocols, and propose a secure and anonymous buyer-seller watermarking protocol. In contrast to early work, our improvement on the protocol's security properties ensures that the design requirements are fulfilled. The proposed protocol is able to simultaneously solve the piracy tracing problem, the customer's rights problem, the unbinding problem, the anonymity problem, the conspiracy problem, and the dispute problem. In the proposed protocol, a buyer can purchase digital contents anonymously but his anonymity can be revoked as soon as he is adjudicated to be guilty by a legal institute, such as civil court.