A very clear summation of the problem. For me, it is beyond believing that the seller was not bound to at the very least produce the books in a good faith way to attempt to complete the transaction.
Let's reverse this, and imagine it was the seller complaining about the buyer: "He made an offer which was accepted, he agreed to pay cash and meet at a given location, finally he said he'd changed his mind and didn't show."
As for the reasonable assumption that there must be some room for people to change their minds, respectfully, that room must be defined very very narrowly.
Also respectfully, you are speaking in generalizations, which is certainly useful and is exactly what the PL Rules are: generalizations.
Here we have a very specific and rather unusual situation, though. I agree the room for mind changing must be defined narrowly ("very very" is a little overkill for a generalization ). But now we have a specific incident. In assessing this and the details outlined, I just do not feel a full deal was ever reached. Thus nothing to "violate".
But it IS very very troubling. I would certainly have no objection to knowing who the seller was.
I feel the opposite, if this was to take place on the boards directly through a sales thread it was:
Book A graded at 4.0
Price $Some amount of money,
Brock then posted pending scans (in this case seeing in hand).
Sorry realized I don't need to sell it that low.
That is why I spoke to specifics. What you cited is not what happened here. Even CC says there was more to discuss when he said "the deal didn't make it that far". The PL Rules are like the OSPG - a guide.
Personally I would love to see copy/pastes of the emails involved in this. No need to include specific names but this needs more than a summation of events by one party. A record of the email exchange would certainly help form a much more informed opinion .
I am not saying throw the seller to the wolves or anything, but sounds like there was a number of books involved in a lot, the grades of the books and the price was agreed upon. Some time later it seemed like the seller decided I will throw the line back in the water and see if I can get a bit more for the books.
I agree more specific exchanges would be more help.