And further if you do insist that the house should be valued at 20% less than you should back it up with some good reasons and data.
All that said, no reason you can't give it a try just good to keep your expectations managed.
If you are serious about buying a home then you will need to pay market price (or someone else will buy it for market price). So if you see a listing and you and your REaltor do your homework and can prove that it is in fact over-priced, go ahead and submit the offer.
In most cases it is unusual to have a home mis-priced by that much.
In real estate, every transaction is unique. There are many variables that determine how much to offer vs. how much the seller will accept. If you have concrete statistics that demonstrate why you are justified in offering 20% below asking, go for it. A good Buyer Agent (who represents YOU and your best interest...i.e. negotiating on your behalf for the best price and terms) will assist you in determining what is a reasonable offer. Of course, only the sellers and their agent knows what their bottom line is...so all the information you present may be useless....you never know unless you try!!! Let me know if you'd like some help! Good luck! Judy