"whenever something is morally allowed for an individual, the moral system must ensure that the individual actually can do it"

More like "whenever one of an inherently-coupled pair of actions is morally allowed, the other of those actions is morally allowed". A moral system which allows one and not the other is self-contradictory.

In this case, a moral system which permits the right to sell a life but denies the right to buy a life self-contradicts; unless you believe that it's moral to trick someone into committing an immoral act (I do not), any attempt to exercise the right to sell one's own life would be inherently immoral because of the impossibility of doing so without causing someone else to perform an immoral act. The only way for the sale of one's own life to be moral is for the corresponding purchase to also be moral.

"we end up with a moral dilemma"

Only if you insist that buying is wrong while also insisting that selling is not wrong. When both are right or both are wrong, then there is no such self-contradiction.