I read an interesting article that goes like this: It really does hurt to sell one of your possessions, independent of how much monetary value you put on it.
I am certainly a frequent victim of this effect; I almost never sell anything, and I find it very difficult to throw things away. Some members of my family* have it even worse than I do. Instead of getting into the neuroscience of this though, I'm going to riff on the broader implications. This result implies that we, as individuals, have a sense of self that extends well beyond our physical bodies, and well into the things around us that make up our daily lives. And, I would argue, also to the people around us, and also to their possessions. This expanded self-identification allows us to empathize well with our friends and families and neighbors, because we literally do feel their pain, or a shadow of it, when something goes wrong or something is lost. If my friend sells his car, I feel sad.
The degree to which my mood tracks the well-being of my car and my computer can be a bit disturbing, frankly, and I think that as we integrate more and more technology into our lives, more of our identities will become digital, and we will identify more with digital 'things.' We already identify strongly with our blogs, our online avatars, our email inboxes, our weighted companion cubes.
Is this effect exploitable? Why yes! Give someone a free gift, where the gift ties them into continuing to pay for your services. If they accept the gift, they will be far more likely to pay monthly fees than to find a cheaper service provider, because parting with the gift will cause them pain. See: cellphones, MMORPGs.
Anyway, I love the idea that our selves are not sharply defined by the borders of our physical bodies, but extend in very real, physiological ways, far out into the world we care about. Overlap in self-identification creates community and shared responsibility. Good times.