When a law bans exchanges wanted by everyone directly involved a number of things happen:
1) The exchanges continue;
2) Prices of the banned items rise and wars to control turf begin;
3) New criminals are created, including many people who are ordinary good people (like colored margarine seekers);
4) New enforcement agencies and staff are created;
5) New jails are built and new jailers are trained;
6) Laws, lawyers and lawsuits proliferate;
7) A new branch of law and its practitioners prosper and support further extension and complexification of regulations;
8) A portion of the entire apparatus of enforcement and punishment is progressively corrupted;
9) New agencies and staff are created to discover, eliminate or suppress the corruption;
10) Many begin to support ever more drastic suppression and punishment;
11) A profitable subliminal partnership emerges unifying the interests of violators and enforcers as the profits from the illegal trade are negotiated and distributed among them;
12) The business engages all of the following: bad people buying and selling, good people buying and selling, police, judges, academics, enforcement trainers and suppliers, prison builders and suppliers, staff to support all of this, journalists to cover it, media organizations to sell the coverage;
13) Completely uninvolved people are caught in crossfires, including taxpayers;
14) The costs of controlling the new flourishing evil continue to grow seemingly without limit;
15) The vast network of beneficiaries of the law applaud and lobby for its continuation, vilifying all opposition;
16) Everyone gets more and more discouraged and inclined to hate all humanity. This list is probably too short.
However all of these bad things may be balanced by the fact that creative people are engaged in producing media based on the things that happen because of the prohibition, and by watching and reading we all learn delightful new things about how the world works. (channeling Voltaire).
It is not enough to simply ban exchanges that have consequences we don’t like. The costs of doing it should be compared with the costs of not doing it. Those costs usually dwarf the costs that would arise from unhindered transactions.