People generally put more value to things which they really own or posses ,so if you have to value two exactly same things and one of them is owned by you then you will put more value to the same thing which you posses or own and this phenomenon is called endownment effect. Reason for this effect is people hate to loose (even when someone pay for that) things they own and material attachment. People generally make irrational decisions as well when encountered with love ,fear or hatred .
So if you encountered with such things try to act logically because sometimes you will get good deal for your product and emotions just fades away with time but material benefits from that last longer.