Can't answer your question without knowing what the house is appraised for or assessed at, at the minimum what similar properties in the neigborhood have recently sold for. It may be a half million dollar house that the bank has 300,000 in, if that's the case negotiation isn't going to get you far.
However, there IS a reason the house hasn't sold in 3 years. Maybe it's the mold, maybe something else. In most states a seller is required to disclose any material defects to a buyer, so they have to tell you about any major problems that exist.