I like to assume that people who are willing to tell lies or take advantage of friendly offers in order to make a profit have some other issues to deal with, whether the troubles are manifested in their daily life or perhaps simply in the mind. It seems better to feel pity than anger towards them. 'Tis rather like people who cut in lines or are generally rude to others - clearly their lives are lacking in some way or another.
I've personally never purchased a bicycle specifically to sell, though I often consider the fact that if I don't care for the bike or it doesn't end up working for me, that I might be able to turn a profit when it comes time to sell. Usually I'm only right about half the time, so I call that fair - especially since I have fun cleaning, tuning, and riding them in the meantime.
-Gregory