The Endowment Effect is a hypothesis stating that people assign higher value to an item once they have obtained ownership of that item.

Once an item has been claimed, foregoing it or losing it feels like a loss, and because humans are loss-aversive, people are willing to pay more to keep an item already in their possession than they are willing to pay to obtain a similar item that is not already in their possession. The term “endowment effect” was first used by the economist Richard Thaler in reference to the under-weighting of opportunity costs, and the inertia introduced into consumer choice processes when goods included in their endowment became valued at higher prices than goods which were not.

Source:

Thaler, R. (1980). Toward a positive theory of consumer choice. Journal of Economic Behavior and Organization, 1, 39-60.