Because if you offer something up at a certain price and the buyer accepts that price, then it's a done deal. Period.
Well, what more needed to be said? "You'll pay? Wait! The price went up!"
The words "but yeah" were right next to them. That's quite a narrow field of vision. Hopefully conversations with you aren't like this too, where once you get the info you need from someone, they cease to exist.
At the very least you were rude for not acknowledging what you "thought" was his last PM. I got a recent offer for a unique book via PM. From the get go I was hesitant so I made it clear that while I was willing to discuss a deal, I may not go through with it.
As the discussion went on I did indeed decide to keep it. But unlike you, we had not even finalized a final price or trade yet. I felt kinda bad but as I said I mentioned up front I may back out.
I immediately PM'd him and informed him of my decision. He was disappointed but understood and asked me to think of him first if I ever change my mind, which I had already planned depending on CGC results.
Courtesy.