Because JMS pinched a lot from the Romans when it comes to the Centauri, my personal fanon for the slave status is that while Centauri can sell themselves into slavery, what happens, if it does, is the head of family sells someone or all of his family to pay debts. This however while legal is regarded as a horrible shame (by other Centauri, and not the slavery as such but the idea Centauri would sink that low), and makes the slaves in question outcasts whom free Centauri would have nothing to do with. Hence Londo's initial shock when Trakis tells him Adira is a slave. (And hence Adira keeping it secret on the station from the start, long before Trakis tells her to steal the files.)

How the people on the station feel: given that Sinclair can lean on Trakis to release Adira from her contract at Londo's request, I would guess that there is some clause in the station's status stating that slavery isn't practiced on the station. Not off station, since B5 has no authority about the customs of other people, of course.