You just need to hire your own work done and have a second appraisal performed on the property from a reputable person who you trust and no try and influence that process.
It will cost a couple hundred but you'll have it at that point. If it is the same as the guy you don't agree with then you have your answer. Nobody likes to lose money on their house but it seems to have happened more in recent times.
Maybe your Realtor is the real idiot when they told you what you should list the house for. You found a buyer so maybe that is not the case... or maybe both Realtors are idiots and the buyers are clueless.
Lot of different scenarios we can break down here, but there is nothing worse than some jackleg home inspector/appraiser/lender, etc. when you are dealing with selling a home.
Get a 2nd opinion.
