Out of curiosity, where would you post this? If it were me I would go back to the listing agent and tell them they need to work this out. Did they offer to get another appraisal done?
I'm sorry you went through this experience. This is one reason of many why buyers should use a buyer's agent and avoid a situation where there could be a conflict of interest. The bottom line answer to your question is that you paid for the appraisal and therefore you can do whatever you want with it. If you wanted to post it online then it is entirely up to you. As the seller or the seller's agent I would not like you very much but eventually they will sell it for what they can get. If someone buys it and for more because they are willing to then your appraisal or a new one will not matter to them. Let's face it some people get emotionally involved in the purchase and they don't care if an appraisal report tells them that it is currently worth less. If the seller isn't willing to drop the price and the buyer really wants it then the buyer will probably come up with more cash to make the deal happen.
Good luck searching for your next house and remember to choose your Realtor carefully. I don't work the Harbor City area but if you need my help maybe I can send you a referral.