Endowment effect
The tendency to value something more just because we own it. People often ask much more to give up an item than they would pay to buy it.
In everyday life
Someone given a free coffee mug asks far more to sell it than they would have paid for the same mug in a shop.
You list your used couch for far more than you'd ever pay for a similar used couch, and you feel insulted when buyers offer the going rate.
Why your mind does this
Finishing what we start is usually a good strategy, since a brain that quit at every bump would never complete anything. Valuing what we already have, what we built and what we worked for keeps us committed and protects our resources. Losing what we have can hurt more than an equal gain helps, so caution around losses often makes sense.
How to spot it
Ask what you would pay for this item if you didn't already own it.
How to counter it
Check what similar items actually sell for, and use that as your price.