This chapter reviews the Proposed Common European Sales Law (CESL) from a number of perspectives: first, whether there is a need for harmonisation in European Internet trading and whether the CESL serves that end; second, whether two national laws on a given subject can co-exist. It uses the example of the evolution of Scottish sales law in relationship to English sales law. Third, it compares the proposed CESL with the UK Sale of Goods Act. In the end, Professor MacQueen sees the CESL as a useful experiment in European private law harmonisation – “It would provide an interesting experiment with which to test the claim that the variety of domestic laws in the European Union is a barrier to the achievement of a single market.” It could serve as a means of filling the need to reduce obstacles to cross-border trading by Internet and small- and medium-sized business enterprises. Because of the CESL’s optional nature, the chapter concludes that no harm would be caused with its passage while significant benefits would be obtained if businesses widely elected to opt-in to the law.