Some of the responses here seem to miss the point of your question: How to negotiate a fair price.
For sake of argument, I'll assume that you're correct that the listing price is $100,000 above the house's current market value. You say the house is worth $200,000; the seller has it listed at $300,000. Those are the facts.
I'll assume that your definition of a "fair price" is one no greater than $200,000. Otherwise, you'd be overpaying and that wouldn't be "fair" to you.
What you have to do is convince the seller that $200,000 is his/her most desirable option. Other options are worse.
That may be difficult to do, because of the large gap in prices. But that's what you have to do.
First, you have to demonstrate to the seller that the real market value is $200,000. You do that with facts and figures. An appraisal. Comps. A CMA from a Realtor. Understand, though, that most sellers think their houses are somehow different. "Oh, my house is 5 years newer than that example you're showing me." "My house is on a 12,000 square foot lot; that one is only 11,500 square feet." "OK, that house has a new kitchen. But mine has better landscaping." Be prepared for that.
Second, demonstrate by showing what the market's like. Perhaps the house in Beverly has been on the market for 180 days, while the average days on market is 65. That sort of discrepancy is the market talking: It's saying the house is overpriced.
Third, make the seller "feel the pain." The seller wants to sell for some reason. Emphasize and underline that reason. "Until you sell this home, you can't move closer to your children." "Until you sell this home, you can't retire to Florida." "Until you sell this home, you can't take a new job in Texas." "Winter's coming. I'm sure you're not looking forward to another winter of shoveling snow and huge utility bills." Find out what the seller's motivation is (that's really not too difficult to do), and work that angle.
Fourth, present a solid offer. You've set the stage: The house is only worth $200,000. And it's hanging like an albatross around the owner's neck. Now you can come along and solve those problems. You can relieve that pain. You present an offer that's consistent with your previous statements and actions. (Odds are that if the property has been overpriced by $100,000, the sellers haven't received any serious offers. Show that yours--while lower than they'd like--is dead serious.)
That's how you negotiate a fair price for the house.
Oh, and one other thing: While your agent can sound the listing agent out about motivation, seller's situation, etc., do NOT--repeat do NOT--quit just because the listing agent reports to your agent that the sellers aren't likely to accept your offer. In fact, a competent listing agent will simply say, "Have your client submit an offer and we'll consider it." And a competent buyer's agent will tell you, "The odds are probably fairly slim of the seller accepting your offer, but we won't know until the offer's made." Point is: You make an offer and see what happens. Worse case scenario: The answer is "no." And you're no worse off than you are today.
Hope that helps.