回复：the owner is the listing agent!!!!! ;-(

Sounds pretty wired. Since tax is assessed based on county record, where the original deed was recorded regarding property boundary. Did the owner subdivided the land without county knowledge or it has been a mistake from the beginning.
Anyway I don't think you are bound by this contract if
the property is proven to be much smaller than it claims.
If you still wants this property, reference the price of property with similar condition and renegotiate the price, often times government assessed value does not reflect fair market value.
As to whether you can sue for damage, depends on if owner knowingly misled info or he is a victim himself.
Anyway, I am not a legal expert, I just had experience from my own purchase. Law differs from state to state, consult either a lawyer or real estate broker.