Why do both the buyer and the seller in a solo exchange pursue their own self-interests?

1.because the parties in the transaction do not plan on doing business together again.
2.because both the buyer and the seller have a high level of dependence on each other.
3.because less hassle means more value to the buyer.
4.because the buyer is confident that, over the long run, he or she will get a fair share of the profit.
5.because both believe the other party would not take advantage of them if given the opportunity