This brings us back to whether a "hold" was implied pending the scans, if not, than it is a continuous negotiation only and no obligation has been created.
That said, given the buyer's last reply posting the I'll take it pending scans, you would think if the seller disagreed he would have replied quickly to clarify, yes?
Agreed.
Absolutely. I totally sympatize with Gambit. If you have no intention to keep the book on hold, or to provide the scans in a timely manner, just say "I can surely get you scans, but I won’t put the book on hold, so that is at your own risk", and that’s OK.
This "there are other books out there" is so "insufficiently thoughtful", so to speak…