Not actively helping souls leave isn't the same as forcing them to stay, if the ones in charge are preventing resurrection spells from working it's slavery, if they are just not resurrecting people themselves, then that's simply the equivalent of not paying for your citizens to immigrate away.
It's the difference between the government not paying for a boat ticket, if you want to emigrate from an island nation, and the government stationing armed patrol boats to prevent you from leaving, them not giving you a ticket is not making you a slave or Serf, them stationing armed patrol boats is, and of course if they put you in chains and force you to work for them, then that's not just Serfdom but slavery.
Which mean the soul trade is just selling body-less slaves, but afterlives aren't inherently slavery, although all the Fiendish afterlives and quite a few others, are the afterlife equivalent of slaver states.