I'd try and play this a little more like a real buyer. I'd be pretty ticked off if I won an auction and suddenly the seller decides not to sell it. Check out the terms of service for ebay regarding sellers retracting items they're selling. If he has broken the terms of service, explain that to him, but don't make any actual threats about contacting them... it would just be implied by the context. Conversely, play dumb and ASK him whether or not rectracting the item for sale is allowed by ebay once the auction is over. Again, you have to seem like a winning buyer who wanted the item he bid on.