I honestly did not have time to navigate through 7 pages, so please forgive me if this was already mentioned.
Since OP is talking about flipping, we can assume this is not for his "personal collection" - which to me is by far the most unethical thing to say when planning a flip.
The fairest way to deal with this is to offer the seller the option to consign the book(S). This really keeps things honest as it is in both parties interest to get the highest value for the book.
If time is of the essence and the seller has a price and has rejected the consignment, then there would be no ethical problem. Both parties are getting what they want.