The Old Testament rules were specific to the Hebrews' situation. God authorized them to enslave the prior occupants of the land that God sent them to possess, and could and did have harsh laws about that. This was a one-off situation. That theology can't be carried forward even into slavery under the Roman system.
And, it's notable that "liars" are named along with the slave traders in Revelation as among the hell fodder -- puts a damper on the theory that this was simply talking about acts of war.
There are at least half a dozen passages in the New Testament supporting slavery. At worst, it seems like a bad idea to go out and capture slaves, but there's no problem owning them.