I would talk with a buyers agent and have them run comparables for you to review to submit the offer. What the seller may accept is still an unknown but that is normally the best way to proceed.
Just make sure when you look at the subject property (the house you want to buy) you are adjusting numbers based on upgrades the subject property has versus the sold properties, and same goes for lot size, # of bedrooms and baths. I doubt highly any of those homes are direct comp's to each other so you need to make sure you take that all into account when pricing the subject home. There is also the chance the subject property is the white elephant on the street which means it is bigger/nicer than the other sold properties so therefore worth more.
We can only give you advice based on the info you provided here so without the address / mls number of the subject property we are limited. Generally speaking a 15%+ discount from list price is pretty steep but if the property is over priced it doesn't matter.
If you need any more help valuing the property please feel free to give us a call or email. Valuing properties is our specialty.
Massachusetts Premier Buyer Broker
What homes sell for on a street and what a home is "assessed" for may or may not be indicative of the home you are looking at as to price.
Two recommendations, get a buyers agent, but more importantly do your own home work and know what the competition is that you are comparing. That will truly tell you the value of what you are looking at is worth.
A new listing you may not have a second opportunity to buy it. A listing that has been on the market, you may have some flexibility. That said any good agent can tell you that a home that has been on for a while may get out of the blue 2 -3 competing offers. No rhyme or reason. Real estate can be a crazy business.
There is a huge difference from a buyer that has to buy and one that wants to buy.
How to value a house is something your agent can do for you. I would happy to help.
Best of luck!