The reality for sellers on eBay is that buyers can find almost any reason why an item is not as described to claim a full refund, with the seller meeting the cost of delivery both ways. This applies whether the item was delivered or collected in person, but not if the buyer arranged for a third party or their own courier to collect.
Don't do anything until the buyer opens a case under eBay's money back guarantee. You will need to respond within three working days to accept the claim - eBay will enable you to send the buyer a prepaid label, unless you and the buyer can agree a partial refund.
When the item is tracked as returned to you eBay will issue the buyer's refund, and charge this to your account. Don't even think about cotesting the claim because the result will be the same - except that you will receive an account defect for failing to resolve the issue with the buyer. Also, if you didn't send a return label eBay may simply enforce the refund and not require the bike to be returned.
There is no point in appealing - eBay takes the buyer's word against the seller's. You can if appropriate report the buyer for abuse of the money back guarantee. If other sellers do the same eBay might at some point restrict the buyer's access to the guarantee.
