Without mentioning a price, let the seller know, that if the buyer decides to pass on the deal, that you're interested in buying the bike.
That way, he knows that he doesn't have to accept less than his asking price, and it still leaves it up to the potential buyer whether the deal gets consummated or not.
Usually, the seller lets the original buyer know, that he's got other interested prospects, and the og buyer ponies up, and the deal goes through.
Sometimes you get the call.
If you get the call, then I feel like you're obligated to follow through, and purchase the item though.