In the United States until 1865 it was legal to own people, sell them, and buy them. This country still has lots of problems but there has been some progress.

I found this at Jerry Coyne's website:

On December 18, 1865, Secretary of State William Seward proclaimed that the 13th Amendment was adopted, prohibiting slavery. Here it is:

AMENDMENT XIII

SECTION 1

Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.

SECTION 2

Congress shall have power to enforce this article by appropriate legislation.