Slaves/serfs make economic sense primarily on a plantation/manor where they can be given use of a small plot where they construct a shelter and raise their own food. They receive little or no subsidies from the planter/lord.

The difference is that serfs are attached to the manor instead of being owned by the planter. Thus, serfs are not sold, but change lords by inheritance, marriage, or other changes in land rights. Europe was so densely populated that serfs probably had little or no cash value. If you had a manor, you already had serfs.