As an economist by trade I agree with you that the term trade-off is inappropriate, because supply and demand are driven by different sources. On the other hand, supply and demand do complement one another insofar as they must both take place at the same price. Thus, the word exchange is very appropriate.

With regard to the term "rapports" the word relationships is entirely suitable, as the price at which the exchange takes place is not always equally beneficial to both side of the exchange. There are a larger number of other relationships to be considered such as price ceilings and floors, as well as tariffs, taxes, and quotas -- to say nothing of an entire myriad of hidden factors included in market structure that are determined by language, culture, and psychological attitude of the market players.