In an exchange there must be an equality to ensure justice. Now modern economics suggests there is always inequality in that both parties engage in the exchange in order to receive a greater benefit than what is given.
In the exchange A is received and B is given. My desire for A is measured in units of B. The desire of the seller for B is measured in units of A. The equality that is achieved for justice is that units of A are equal to units of B in some fashion. This is seen in that the exchange takes place and men do not suffer an injustice willingly.
However agreement of price between parties is insufficient to establish justice since justice is real and objective. This is seen in cases when the price becomes unhinged from the reality of the objective value of the things.
This occurs manifestly in cases where a non-existent is sold or something is charged for twice as in usury. Selling the non-existent bridge is certainly deceitful but also unjust in the attempt to exchange for nothing. Moreover there is a clear disorder in the desire of the buyer in that he desires nothing or rather something simply fictional.
Also in the case of price gouging the price becomes unhinged from its objective worth. Since one does not suffer injustice unwilling they agree to it out of necessity rather than an concordance of wills.
Another example is a superstition when a man thinks something is more power than it actually is. He will pay more than its actual value and to take advantage of this superstition is unjust.
This also suggest the absurd prices charged at movie theaters and baseball stadiums are also unjust in that they are unhinged from reality.
In these examples there is a disordered desire in various respects and in either party.