eBay aren't going to get involved here if you gave the seller your collection code, which obviously confirms that you've collected the bike.
I can't see another route to getting your money back, to be honest.
If you paid with your debit card and try and issue a chargeback via your bank and the seller has the collection code it may be viewed as a fraudulent claim by them, which isn't something you want.
Has the seller stopped contact with you?