Why not ask another EA to value the property? If Bank will lend to you it will be on the basis of their valuers value so why not ask the bank for a list of their valuers? You can offer then what you think is fair value or even a much lower value. The vendor can choose to do business with you or not.
After that, remember that you don't as a prospective purchaser always get to decide the price. The Vendor may be willing to negotiate on price. Or not. Not everyone needs to sell.
People tend to forget that.
mf