It seems to me that a 100%, 500+ feedback seller may be honestly telling you that the owner is refusing to turn over the goods, so he can't deliver. Maybe I'm too softhearted, but unless the product was something I absolutely had my heart set upon, I don't think I'd push someone through a sale they really didn't want to complete. Dealing with bikes and bike stuff on ebay has always been just fun for me, and I don't get much fun out of leaving someone feeling like he got the short end.
One thing likely to close a tough deal, though, is to let it be known that you're willing to walk away. I wouldn't ante up another dollar, but I'd probably ask if they wanted to give you a refund so they could re-list it or put it on CL if they thought they could get more. If all that gets you is a refund, that's not the end of the world.