You're completely wrong. They were sold exactly the same way. It is true that slaves could, with the permission of their owners, earn money and even buy their freedom in some cases, but that was neither required nor even the norm throughout the majority of Roman history. It is true that some slaves became enslaved as a result of debts, but that was hardly the only route to slavery. Read Caesar's The Gallic Wars, he enslaved entire towns and tribes, the Veneti for example.