Which argument refutes the theory of Just price?

Thinking experiment that proves that seller couldn't be be pressurized to lower price

Failure of socialist economies in East Germany, North Korea, Cuba, USSR

Just price was proven to be the most efficient theory in practice

Which types of monopoly exist (if there is no mistake):

Government, Coercive, Efficient

Necessity, Efficient, Government

Spontaneous, Coercive, Necessity

Creating modifications of the same product and selling them at different prices not based on costs differences

Deliberately damaging their already produced goods to create versions with limited functions or low performance

Charging different prices for various units of the same good that do not reflect marginal costs differences.