Time is Money

Whether buying or selling a home, the Offer To Purchase is the starting point for making the sale go through.  If the sellers do not accept the offer outright, they may make a counter-offer, which the buyers may likewise accept or counter again.
In the interest of speed and success, it's best to keep counter-offers to a minimum.  If you are trying to sell with urgency (and who isn't?), weigh the buyers' offer against your need to move quickly.  Perhaps the value of the concession is quite small against the profit you'll see upon a sale.
If you are asking $200,000 and receive an offer of $196,000, that's equivalent to just 2% less, which is like offering $.98 instead of $1.  Similarly, buyers must also be realistic about the possible costs of "over negotiating" in today's rapidly changing economic atmosphere.
If you balk at the sellers' counter-offer now, and decide to walk away and begin your home search over again, you could be facing higher interest rates and/or rising home prices.  Today, time literally is money, and the longer you postpone your purchase, the more it will likely cost you.
Before making (or accepting) an offer, discuss the offer and counter-offer process with your representative, so that you know what to expect and can be more prepared to see the deal to a successful close.