Because all transactions are unique there is some flexibility in the marketplace. The amount of flexibility depends on local conditions.
Suppose you're selling a townhouse. There have been five recent sales of the model you own and those sale values have ranged between $200,000 and $210,000. These recent sales give you an idea of how your home might be priced. In a strong market perhaps you can ask for $210,000 or a little more. If the market has slowed, $210,000 may be a reasonable asking price, but perhaps more than the final sale price.
Here's another example. Imagine that you live in a community of Victorian-style homes, most of which were built in the 1920s. All the homes are different in terms of size, condition, modernization, style and features. In such a neighborhood, an average sale price is just a statistic without much practical meaning. On a single block one home may sell for $400,000 while another is priced at more than $1 million. For this reason, the average price may be outrageously high for one home and staggeringly low for another, making this figure unsuitable as a price-setting standard.