I would as well. Turning this around if I asked someone if he would take $100 in a pm and he said yes then sold it to someone else I'd be upset as well.
I was very guilty of this in the past.
The asking price was a "no" for me but I'd PM sellers just to see what their wiggle room in price was. Sometimes you'd get a "sorry, can't do it" but sometimes you'd get - "I can do that. Just mark the thread."
I was doing it just to give me something to mull over. I've definitely toned it down as I regretably annoyed some sellers.
But I don't think inquiring about wiggle room in price is the same as if the seller would sell it at that price.
Just to give you something to think about, you can add me to the list of sellers this type of action would annoy. In my mind, it'd be like if you went to a convention and you walked by a booth:
You: Hey, would you accept $100 for that book?
Dealer: Sure! *turns around to start wrapping up the book*
You: Neat! *keeps on walking*
You definitely don't come off well in that situation. Again, just something to think about.
This is definitely the reason I ask people to post the in the public thread to make the sale official once PM negotiations have concluded. I usually follow it up by telling them the book is still up for grabs until it is claimed. Some people prefer to remain anonymous and ask me to mark it as "Sold via PM", which I'm happy to do, but at least they've confirmed without a shadow of a doubt that they're committing to buying the book.
+1