It would be possible! If your character has something of value that could be reasonably sold for a slave, then they could make that trade. It can be successful or go south, it's up to you. They could also try to fight them. Whether that's successful or not, again, depends on what kind of slavers you decide your character is up against. They can be super powerful, hightech slavers, or not.

It definitely wouldn't be formal. People show up all the time trying to get slaves back. Slavers get killed all the time. It's mostly just not getting caught by the wrong people or being overpowered.