So long as slavery was legal “buying” the freedom of slaves within the USA (even if all slaveowners were willing to sell at some price) would only mean a similar number would be imported again. Under the conditions of the time (i.e., views in the south) no constitutional amendment or even congressional legislation could have outlawed slavery without first fighting the civil war. The south was determined to maintain a balance of “slave” and “free” states precisely to guarantee that slavery could not be outlawed. The conflicts in Missouri and Kansas etc. over slavery were intense and bloody. The idea that anyone as President or in any other position could have merely waved a magic wand and purchased the freedom of all slaves while simultaneously arranging a permanent abolition of slavery is a fantasy. Ron Paul is ignorant of the basics of antebellum history and politics.
The Constitution sunsetted the trade (importation) of slaves back in 1808.
Some think they would’ve smuggled them in, but to that extent to cover all the freed natives? No way. Would never have gotten away with that.