For what it's worth I'd be inclined to try raising a dispute on the basis the saddle is "significantly not as described". The seller might be technically accurate in saying he only put 40 miles on it but saying it's "used" doesn't get him off the hook - it just says that he used it. If he put it on his bike, rode half a mile and took it off again it would be "used". "Only 40 miles" suggests the thing is nearly new as opposed to being some old knackered thing that he only used for a short time.
The seller has either misdescribed it or described it using deliberately misleading terms. From what people complain about on here ebay and paypal seem to side with buyers to the point of letting crooked buyers take honest sellers for a ride so you might be in with a good chance.
Maybe you'll be out the shipping back, maybe you won't. Still beats being out the full amount you paid.
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