It doesn't matter what you can prove. If the buyer bought the property at an auction and probably got it under market value, he can set any price he thinks he can get for it as the price to sell it for. You can also offer him whatever you feel the property is worth. Either he will accept, reject or counter on your offer.
Have you looked to see what property in the area sells for. If most homes sell for $100,000 and you pay what he is asking or say $70,000 and have to put $20,000 into the property to get into good shape, you are still $10,000 ahead in property value.... more