A counteroffer is a response from the seller to the buyer, proposing different terms or conditions than those initially offered. The validity of a counteroffer depends on the specific terms stated within the counteroffer itself. Generally, counter offers have a specified expiration date or timeframe within which the buyer must respond. This timeframe can vary, but it's usually a few days to a week. If the buyer accepts the counteroffer within the stated timeframe, it becomes a legally binding agreement. If the buyer does not respond within the specified time, the counteroffer is considered null, and negotiations may need to restart.