I made an offer by PM to another Pinsider for an aftermarket mod (not for a whole game) which he accepted by PM.
I PM'd him back asking for instructions on how I should pay for the item (by PayPal or whatever).
He later PM'd me that someone else had since offered him more money and he had agreed to sell it to him.
I raised my offer quite a bit and reminded him that he had already accepted my initial offer.
He replied that he was now losing $60 by not taking my raised offer, but he would not go back on his word to the other guy.
I then reminded him that that was exactly what he was doing to me by going back on agreeing to my original offer.
He replied that I was correct and apologized to me for not giving me the chance to raise my offer, but still sold it to the other guy.
I felt his apology did not make things right (and I really wanted the mod), but there was nothing I could do about it.

I am not naming the Pinsider or identifying the mod, since he did apologize at least.