The defining feature of slavery is the involuntary part of it.

Quick googling (could be wrong), thousands bought freedom, approx 4 million were enslaved. It would have been uncommon, ballpark 0.1% (which still seems high to me. The issue is a very large denominator)

Slavery was around long before the US existed. In Ancient Rome it was much more common for slaves to buy their way out of slavery..

Sure, but the context is America. It would have been helpful to make clear you are referring to other systems of slavery (for which i would still want to see data for).

Again, the resources I found in a quick search state about 5M slaves in Rome, and buying freedom was uncommon.