Unreasonable expectations can ruin all kinds of relationships, including ones between prospective home buyers and sellers.

That’s the word from real estate professionals who have watched deals fall through because one side or the other made unrealistic demands. To be effective, they say, negotiating a home sale must be a win-win process. In other words, both the buyer and the seller need to feel they have won.

Start by negotiating a home sale in good faith. It’s reasonable in today’s slower housing market for a buyer to start out offering 5 percent below the asking price, says Matt Dolesh, an agent in Oakton, Va., for Prudential Carruthers REALTORS®. Even 10 percent below the asking price could be reasonable, he says, but it depends on the situation and how close the asking price is to recent comparable sales.

Buyers and sellers generally agree on a price 1 or 2 percentage points below the asking price, Dolesh says.

“You need to put a reasonable offer in that shows you have room for negotiation, but you also cannot put in an offer that’s 20, 30 or 40 percent lower than the asking price,” he says. “That puts a bad taste in people’s mouths. People have ownership and pride in their home, and a low-ball offer can almost insult them.”

Experts say buyers should be prepared to give up something they want in exchange for the sellers giving up something they want. Both buyers and sellers need to be prepared to suggest options and consider options proposed by the other side.

Being flexible about such things as contingencies, closing dates, alternative financing and repairs signals you will be reasonable and is likely to keep negotiations focused on the positive.

“You want to have an offer that protects you, but you also don’t want to nitpick at things,” says Dolesh. “Keep the contingencies to a comfortable minimum.”

Negotiations can sometimes fall apart over the home inspection, which comes after the buyer and seller have agreed upon a sales price. “Don’t present a long list of things to be repaired, just five or six things that will really make a difference,” he says. Besides signaling that other parts of the negotiations could get bogged down in details, a buyer’s insistence on having minor problems fixed can offend sellers the same way a low-ball offer can.

Other key ways to negotiate without torpedoing the deal:

• Put down a large earnest payment to show you are serious about buying the home.
• If you’re a seller, consider offering to pay all or part of the closing costs.
• Keep emotions out of it and be ready to make a decision.

Finally, realize that some deals aren’t meant to happen. Dolesh recalled a deal in which a seller decided at the last minute to reject the highest offer. Her late husband had been killed serving in the military, and she decided to sell to a lower bidder who had military ties.

“Those are the kind of things you can’t control,” he says