This is called the endowment effect.
Once something becomes “mine,” people often value it more than they did before owning it.
Nothing about the object has to change.
Ownership changes the judgment.
A simple mug can demonstrate it
Imagine you are given a coffee mug.
A few minutes later, someone offers to buy it.
People often want more money to give up the mug than they would have been willing to pay for the same mug before owning it.
The mug is identical.
The reference point changed.
Ownership can turn giving up into a loss
Before you own something, buying it means deciding whether the gain is worth the price.
After you own it, selling means giving something up.
Because losses can feel especially important, the object may suddenly seem more valuable.
Where does this show up?
Houses.
Collectibles.
Old furniture.
Cars.
Personal projects.
People can become attached to things simply because the items have become part of what they consider theirs.
Does sentimental value count?
Yes, although sentimental attachment can add another layer.
An object tied to memories may genuinely mean more to the owner than to anyone else.
The endowment effect can still influence ordinary items even when no deep emotional history exists.
Why can selling used stuff be difficult?
Owners know what they paid.
They know how useful the item was.
They know its history.
A buyer mostly sees the current object and the current market.
That creates a gap between personal value and outside value.
Is the owner always wrong?
No.
Sometimes the owner’s price is reasonable.
The point is that ownership itself can influence valuation.
Recognizing that effect helps explain why two people can look at the same old chair and see very different numbers.
Nothing about the mug changed.
Ownership did.
A related piece of human behavior appears in Why do losses feel worse than gains feel good?, where the brain handles another everyday judgment or response.
And that’s the short explanation.
Related explanations: Why do losses feel worse than gains feel good?, Why do we keep going because we already spent money?, and Why do we want things more when they are scarce?