It's called the "endowment effect" and applies to most things we sell
ourselves:
http://www.npr.org/2013/12/26/257394308/are-we-genetically-inclined-to-be-ma
terialistic

Excerpt:

"Once we own something, we start to value it, we want to hold on to it.
Researchers have long believed the endowment effect has biological roots.
Deep in the brain, it feels worse to lose something than it feels good to
gain something."

Definitely worth listening to the whole piece.

Stuart

'85 Westy (I'll take $25k for it!)