We're getting a little far afield, but since you're bringing it up, here's my .02. A second, a minute, a day...time period before remorse is irrelevant, unless the deal has a specific cooling off period. Likewise, public nature vs private. Likewise, a PM "I agree" vs a click of a button vs a post of a graphic.
What is in question is what moving parts of a deal are necessary to finalize before one can say "a deal has been struck" and which entitles parties. Clearly price is one aspect. In your example above, that part imho is done and done. Which doesn't mean we have a deal in total. There's manner of compensation, time element, right of return, etc., all to be agreed upon beforehand.