I imagine some of them manage to go on to lead a somewhat normal life, but a lot of them become pirates. The founder of the legendary Sun Pirates was a former slave named Fisher Tiger. One of my crew escaped being sold into slavery as well.
[Awkward wording because Law refuses to take credit for saving Jean Bart because he's weird and cagey like that.]

I think it's a natural course of action if you have the will for it. A lot of people turn to piracy because they've been wronged by the World Government, even beyond those that were enslaved.
[That last sentence is said with a particularly stony tone, as Law scowls down at his open palm before tightening it into a fist. He won't say it, but it doesn't take a lot of intuition to suspect he's speaking from this on a personal level.]

But that sort of attitude can be their downfall, you know? To underestimate your enemy. That's why those Precursors lost to our crew.