Should all voluntary contractual exchanges be allowed among consenting adults?

I briefly mentioned the following hypothetical exchanges during our last class:

1. A complicated medical procedure has been discovered that cures a rare form of cancer in patients who would otherwise certainly die. Staff shortages make it impossible to treat all those who would benefit, and the hospital has established a policy of first come, first served. Ben, a wealthy patient who is at the bottom of the list, offers to pay Aisha, a poor person on the top of the list, $1 million to exchange places. If Aisha dies (which is very likely), then her children will inherit the money. Aisha agrees.
2. Melissa is 18. She has been admitted to a good university but does not have any financial aid, and cannot get any. She signs a four-year contract to be a stripper on the Internet and will begin work when she is 19. The company will pay her tuition fees.
3. You are waiting in line to buy tickets for a movie that is almost sold out. Someone from the back of the line approaches the woman in front of you and offers her $25 to exchange positions in the line (he takes her position in front of you and she takes his at the back of the line).
4. Apolitically apathetic person, who never votes, agrees to vote in an election for the candidate who pays him the highest amount.
5. William and Elizabeth are a wealthy couple who give birth to a baby with a minor birth defect. They sell this baby to their (equally wealthy) neighbours and buy a child without any birth defects from a family who needs the money.

. An individual with an adequate income, decides that he would like to sell himself to become the slave of another person. He finds a buyer willing to pay his asking price. The aspiring slave will use the money to further his children's education.

In an essay, please select one (1) of the hypothetical exchanges and answer the following three questions:

1. Explain why the transaction described produces mutual benefits (that is, explain how it is a Pareto improvement over not allowing the exchange)
2. State whether or not you personally approve the exchange and explain why
3. Decide, regardless of whether you do not approve, whether you think the transaction should be prohibited and explain why

Please assume in each case that the people involved are sane, rational adults who have thought about the alternatives and consequences of what they are doing.