Even absent contracts (which exist to enable collusion) the iterated prisoner's dilemma and its generalizations seem to be solvable by tacit agreement (in the two party case, by tit for tat).
The only people who maintain that markets have the perfect competition property are those who define markets this way. Their existence is left as an exercise.
The only people who maintain that markets have the perfect competition property are those who define markets this way. Their existence is left as an exercise.