How trading relationships usually work better if the two sides possess equal strength.?

Answer 1

I will answer this in the context of international trading partners.

The ideal relatrionship is that both sides have something in excess of what they need, but is scarce compared to the need of the other. They enter into a trade transaction where the value of the goods traded are equal, thus maintaining a balance of trade.
Obviously in the cases of international trade many goods are traded between the two trading partners. The objective of each trading nation is to maintain a balance.

If one trading partner is stronger than the other, the weaker partner will likely have an imbalance of trade that is in favor of the stronger partner. This imbalance requires that the difference by offset monitarily. This could involve direct payment in a currency like gold; it could mean a loan against the deficit;or it may result in the weaker partner selling an asset such a real estate to make up the difference.

In the case of the US, we are stronger than any other nation on Earth, however, we are typically a weak trading partner. this results in the US trading at deficit to many nations. We issue bonds to offset the deficit, which creates debt. (some term this as "leveraged").

This has resulted in many nations competing to manufacture the goods that we consume. They suffer a lower standard of living than the US. It also means that the US economy influences all economies on Earth.