[Matt Monson]

You need to be realistic. You aren't going to get it. You are asking the seller to rebuild your engine for you for free. Your demand letter is super aggressive and likely why the seller hasn't replied. The reality of situations like this is that unless you are willing to sue the guy, you have to figure out some compromise number that you both can live with. Backing the guy into the corner like you did will likely only result is added resistance and push back. People are like Chinese handcuffs and if you want to slip out of this mess you need to be a bit more gentle.