Economists often assume that markets are inert, that they do not affect the goods being exchanged. But this is untrue. Markets leave their mark. Sometimes, market values crowd out nonmarket values worth caring about.
When we decide that certain goods may be bought and sold, we decide, at least implicitly, that it is appropriate to treat them as commodities, as instruments of profit and use. But not all goods are properly valued in this way. The most obvious example is human beings. Slavery was appalling because it treated human beings as a commodity, to be bought and sold at auction. Such treatment fails to value human beings as persons, worthy of dignity and respect; it sees them as instruments of gain and objects of use.