Serfdom
A manorial lord could not sell his serfs like Romans sold slaves.  However, serfs were legally tied to the land they worked on.  If their lord sold his land, his serfs were sold along with the land.  Serfs could not sell the land they lived on, and could not leave the manor without their lord’s permission.  Often, they needed the lord’s permission before they could marry someone who was not also a serf for that lord.