Scripture presents sexuality as unifying, powerful, affirming and mysterious. Sexual sin is in a category unto itself (1 Cor 6:18). It's noteworthy that the final state of willful rebellion against God is characterized by homosexual sin (Rom 1:24-27).
In the highly recommended book "Sex and the Supremacy of Christ" (here), John Piper asserts that
"sexuality is designed by God as a way to know God more fully ... the ultimate reason (not the only one) why we are sexual is to make God more deeply knowable ... There is a reason Scripture uses sexual terms to describe the relationship between God and his people - both positively (when we are faithful) and negatively (when we are not.)"
In other words, God empowers humans (created in His image) with sexuality so that we might have a hint of the joy and ecstasy of what it is to know Christ supremely. The reason then that Scripture warns against the misuse of sex is that it distorts the true knowledge of God. Remember that "knowing" someone in the fullest sense is defined sexually (i.e., Gen 4:1; Matt 1:24-25, etc.)
Why then cannot sex ultimately be bought or sold?
Because nothing sacred can ultimately be purchased or bargained for.