And they shouldn't. For one thing this would be an anticompetitive move against the other members, and for another, once you import something into one EU country it's really hard to stop it from going into the other countries. So the EU has to approve the regulation of such an agreements...

Otherwise, even without malicious intent on part of a member state, a country like china would just go shopping around the 26 member states and ask who's giving them the best conditions and then do all their trade through them...