I cannot seem to reject the return.
You can't. Only eBay can decide the outcome, but they do so without seeing the items involved and generally accept the buyer's word against the seller's for the item condition.
You will need to accept the return, when eBay will enable you to pay for and send the buyer a prepaid return label. Once the bike is returned with tracking proof of delivery, eBay will issue the buyer's refund and charge you for it. If you don't, eBay is liable to enforce the refund anyway, and not require the buyer to return the bike.
Hopefully you will receive the bike back in the condition sold. If not, you can take this up with eBay. Unfortunately, we often hear from sellers who claim that their items were returned damaged, or even switched for a different one, but were still required to refund the buyer. In such cases eBay has explained that as they never see the items either before or after delivery they were unable to determine who has the "valid case". In this situation eBay's policy seems to be to give the benefit of any doubt to the buyer. In the user agreement you gave eBay the power to make these decisions, and agreed to accept them.
If you still believe that the buyer abused the money back guarantee you can report this. It won't help your own situation, but if other sellers report the same eBay may eventually ban the buyer from opening further cases.
It's important for sellers to understand the risks of selling here, especially the risk that buyers may abuse the money back guarantee.
