What term refers to a tag worn by slaves who were for sale?

The correct answer is the term "titulus." In ancient Rome, a titulus was a label or tag that would often be attached to the neck of a slave who was for sale, indicating their price and other relevant information. This practice highlights the commodification of slaves in Roman society, where individuals were often treated as property rather than people.

The other terms refer to different concepts within Roman culture. "Vilicus" describes a steward or overseer of a farm or household, responsible for managing slaves and laborers. "Peculium" refers to the savings or personal property that a slave could accumulate, which was often legally recognized and could allow them some measure of autonomy. "Vicarius" generally denotes a deputy or substitute, often in a legal or administrative context, rather than specifically relating to slavery or the sale of individuals.