It comes down to personal integrity in a business transaction - the operative work being personal. Its evident that this runs the gamut even among pinheads, and probably differs situationally. I've driven 6 hours after confirming a deal that morning only to have the seller literally ask what I was doing there when I arrived, and inform me that the table was sold hours before. I left him with a few choice words before i started the 6 hour drive back - empty handed and fuming the whole way.
If I make a deal with someone, i live up to that deal irrespective of what comes along. There are ways and ways for one to rationalize not doing so. The rationalization will always be sound to one party and not so much to the other.
Ultimately I just try to learn from the experience, move along to the next, and try to protect myself from the lack of integrity that seems more prevalent nowadays.