He was buying up slaves to take to New Orleans.

So he took me and my little family to the work-house, to be kept under lock and key at work until he had bought up as many as he wished to take off to the South.
The work-house of Louisville was a very large brick building, built on the plan of a jail or State's prison, with many apartments to it, divided off into cells wherein prisoners were locked up after night.
The upper apartments were occupied by females, principally.

This prison was enclosed by a high stone wall, upon which stood watchmen with loaded guns to guard the prisoners from breaking out, and on either side there were large iron gates.
When Garrison conducted me with my family to the prison in which we were to be confined until he was ready to take us to New Orleans, I was shocked at the horrid sight of the prisoners on entering the yard.
When the large iron gate or door was thrown open to receive us, it was astonishing to see so many whites as well as colored men loaded down with irons, at hard labor, under the supervision of overseers.
Some were sawing stone, some cutting stone, and others breaking stone.
The first impression which was made on my mind when I entered this place of punishment, made me think of hell, with all its terrors of torment; such as "weeping, wailing, and gnashing of teeth," which was then the idea that I had of the infernal regions from oral instruction.

And I doubt whether there can be a better picture of it drawn, than may be sketched from an American slave prison.
In this prison almost every prisoner had a heavy log chain riveted about his leg.

It would indeed be astonishing to a Christian man to stand in that prison one half hour and hear and see the contaminating influences of Southern slavery on the body and mind of man--you may there find almost every variety of character to look on.