What kind of feedback/reviews has the seller received from previous customers?
Now that you have involved Amazon by raising an official dispute claim then the seller should stop trying it on and do the right thing, I expect Amazon will be quite draconian with errant sellers as they won't want the Amazon brand to be damaged. I recently had a small dispute with a seller over a £6.99 item that never arrived. They asked me to check with neighbours and the local sorting office. I pointed out that they initially claimed to send it via Hermes! I waited a reasonable time and they then claimed to not have anymore in stock so would offer a refund. The refund never came despite a nudge or two which resulted in additional questions like could I confirm details/addresses etc, all stuff they already had. I threatened to open an Amazon dispute and hey presto, a refund in my account within 24hrs. I guess they never had the item to send but thought I wouldn't bother chasing a relatively small sum if they made it awkward to get a refund. How wrong they were!
I should think you will get resolution through Amazon.
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Not as easy as you think. They insist you supply a report from competent person in this case a cycle mechanic. AND you keep the bike to provide proof. I am comparing with similar problem I had.If he bought it with a credit card he could get a refund on that.