I think DL42 has a valid point, the thread didnt say "taking offers" so IMO it is courtesy to reply and deal with the first person who pm's you with interest and then if you cant come to a deal then you deal with the next person.
I agree it's courteous, but no seller of any item has to sell something (even if they advertise it with a price) to any particular buyer, it's called an invitation to treat. Also this is less than that, all he did was say he'd pulled it.
On this site from my understanding it might have been discourteous but well within the rules. Personally if it was a sale thread with a price I think the person selling should ethically sell to the first person who says they will buy it.
So I agree, but disagree