When I think of slavery I usually think of working for free, not H1B tech worker salaries which are insanely high on a global scale. It's also something people actively sign up for.
Worth reading about https://en.wikipedia.org/wiki/Forced_labour, it's not just the practice of chattel slavery, and various forms are still legal in the US.
Again, the resources I found in a quick search state about 5M slaves in Rome, and buying freedom was uncommon.