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It's not necessarily better. Kinoko have a range of other colours and sizes in stock, if someone bothers to look around their site. Yeah P&P isn't free, but with that you are buying from a reputable seller, with more buyer security and the option of returns / exchanges.
Even the mention of 'falling out the back of a lorry' (with the stock images and righteous indignation dripping from the reply) I'd be inclined to proceed with caution.
He's not trying to make you look like a thief. He's probably just arguing that your deal might not look so keen to someone idly googling. I doubt he was trying to stab you in the back. You've now set out all the ways in which your deal is better, so maybe all's well that ends well, eh?