I have ran into this situation for me as a seller a couple times, once on Racing Junk I posted a fuel injection bug catcher that I got in a package deal of parts and took a guess at what it was worth and I told the first caller I would sell it to him and while I am looking into shipping I get 3 other calls on it and get offers more than double my asking price but I told them all that I promised it to the first guy.
I feel my word is a good as a handshake so I go through with the deal knowing I am taking a loss and ask him to email me when he gets it.
So he emails me a few days later and says it looks great so I email him back and say if you are happy with the deal my PayPal takes donations figuring maybe he might throw me a $20 or something since he probably saved a few hundred dollars.
His response was "If you wanted more money for it you should have posted it in an auction instead of a set price"
I didn't bother to respond but I'm thinking really, this is RacingJunk not Ebay I could have backed out and reposted it for more.