It's not a deal until you both "shake on it" as it were. Whenever I make an offer, I don't consider it a done deal until we've both said okay. So no, I wouldn't say you're on the hook with the fellow.
I usually try to phase my offers as a question, just to make it a bit more clear that it's not final yet: "Hey man, would you accept $120 for your Captain Carrot #16 4.5 GLOD?"
The bolded part really bothers me.
When you make an offer, you are extending yourself and basically saying you will do it if the other party accepts. That is not hard to understand.
What really bugs me is when someone says "Would you take $65 for that?", and the seller accepts, then the guy says "well let me think about it". Or similarly after the seller accepts, the guy says "would you take $50 for it?"
If you make it clear that it's not final yet, then good for you, but if you don't make it clear, then dooshy for you.