To answer your question, Brumby - yes, the seller hasn't actually performed his/her side of the bargain, and thus the buyer is prevented from paying. In that case, negative feedback would be fair.
The seller's done the wrong thing on several fronts.
1. By all accounts, it seems he's dropshipping without having stated in the listing that he was dropshipping. Not only is this unfair to the buyer, it's an eBay policy violation.
2. He's appallingly bad at communicating. The buyer deserved to be informed of the purchase status rather th an left hanging without replies.
3. He's treating eBay purchases as "orders". He is heading into seller non-performance territory on a bomb-rigged bus.
4. He is trying to off-load the blame onto someone else - his supplier? - by saying "I'm being stuffed around."
This is not responsible seller behaviour. It's not only not acceptable on eBay, but if a seller in a shop communicated & behaved like that, he'd not win any customers.
Like BNWT, feedback appears largely meaningless to me - and it does indeed give complete knot-headed numpties the opportunity to hand out silly or nasty comments willy-nilly. There's no responsibility taken for one's words and the feedback given. It is completely unlike real-life situations and any sort of real-life shop transaction.
In real life, if a customer behaves like a twit or a criminal, they can be escorted from the premises. If the customer tries to bargain for discounts at the counter on the basis of not being happy with the range of goods in the shop, or not happy with the shop location, or not happy with the colour of the lighting in the shop, or not happy with the total price, or not happy with the hairdo worn by the staff member - and threatens to bad-mouth the shop if a discount isn't given, or to spraypaint graffiti on the shop stating, "Liar! Book not as described - NOT funny and I DIDN'T laugh until I was sick. REFUSED to give refund! Bad seller! Avoid, avoid!" - well, the shop staff would have the option of
a) trying to calm down and soothe the customer, and proceeding with the sale (no discount given, though, of course!);
b) telling the customer to leave the premises and refusing to sell to such an unstable individual;
c) calling the police on the basis of a threat to commit criminal damage having been made.
Whoops - must rush! Mother's Day lunch - have to wrap the present and get ready!