In America, the Civil War pretty much ended slavery (although it was actually the Thirteenth Amendment to the United States Constitution (December 1865) that made it formally illegal to own slaves).
Anyway, that would be the legal obstacle preventing you from buying Justin Bieber.
There is one other issue:
Mr. Bieber makes around $55 million a year according to eOnline).
So, assuming he earns that much every year for the rest of his life, and considering the average life expectancy of a US male is 76 - buying Justin would cost something in the region of $4,180,000,000!