eBay has a system for conflict resolution. I have used it and found satisfaction. I do a number of eBay transactions and have had only one problem. Just write the guy and ask him why he didn't place a reserve on the item. You won the bidding fair and square and should receive the bikes per the deal you negotiated. Don't get into a feedback pissing contest until you use up all the options through eBay. This is why I always use PayPal for this reason.
Also, $50 is a reasonable cost to ship a bike. He may have invoiced you automatically which would have included the shipping cost. Try and communicate with the seller patiently to resolve this.