Sometimes phrases mean something beyond the combination of the dictionary definitions of the words in the phrase. Free market does not mean murder for hire is ok. That's pretty much never been the definition intended by anyone that's used the phrase other then people trying to establish a straw an to attack.
I absolutely agree that sometimes phrases mean something beyond the combination of dictionary definitions of the words of the phrase. I've noticed that it's not uncommon for a large number of people to incorrectly use a phrase. That's why I've already provided a dictionary definition of the whole phrase
'Free Market' so there can be no confusion. I can even provide a few more if you would like:
Free Market - an economic system in which prices and wages are determined by unrestricted
competition between businesses, without government regulation or fear of monopolies. (http://www.dictionary.com/browse/free-market?s=t
Restricting slavery is restricting competition in the market place.
Free Market - an economy operating by free competition
Restricting slavery means that there is no free competition when selling slaves.
Free Market - an economic system with only a small amount of government control, in which prices and earnings are decided by the level of demand for, and production of goods and services
There is demand for slaves (see: the many places around the world where people are still commonly used as slaves - North Korea, Qatar, India, etc.) and production of slaves is available. Most governments exert a huge amount of control over this area of the market to prevent any slavery transactions from taking place though. Certainly not a free market.
Free Market - An economic system in which prices are determined by unrestricted competition
between privately owned businesses. (https://en.oxforddictionaries.com/definition/free_market
Restricting the sale of slaves restricts competition in the market.
If you check closely, not a single definition of the phrase 'Free Market' prohibits selling people into slavery. It is inconsistent to yell 'Free Market GOOD' and then argue for limiting slave trade by the government and against limiting the sale of say handguns by the government. In both cases a 'Free Market' is being denied to the people. You appear to want a limited market, limited to things that you personally feel should be allowed.