Market Value is the estimated amount for which a property should exchange on the date of valuation between a willing buyer and a willing seller in an arm’s-length transaction after proper marketing wherein the parties had each acted knowledgeably, prudently, and without compulsion.
The definition arises from the Australian compensation case Spencer v. The Commonwealth of Australia (1907), 5 C.L.R. 418 in which Griffiths, C.J stated:
“Bearing in mind that value implies the existence of a willing buyer as well as of a willing seller, some modification of the rule must be made in order to make it applicable to the case of a piece of land which has any unique value. It may be that the land is fit for many purposes, and will in all probability be soon required for some of them, but there may be no one actually willing at the moment to buy it at any price. Still it does not follow that the land has no value. In my judgment the test of value of land is to be determined, not by inquiring what price a man desiring to sell could actually have obtained for it on a given day, i.e., whether there was in fact on that day a willing buyer, but by inquiring "What would a man desiring to buy the land have had to pay for it on that day to a vendor willing to sell it for a fair price but not desirous to sell?"”
In the English professional negligence court case Singer & Friedlander Ltd v John D Wood & Co  2 EGLR 84 Watkins J stated:
“The valuation of land by trained, competent and careful professional men is a task which rarely, if ever, admits of precise conclusion... Often beyond certain well-founded facts so many imponderables confront the valuer that he is obliged to proceed on the basis of assumptions. Therefore, he cannot be faulted for achieving a result which does not admit of some degree of error. Thus, two able and experienced men, each confronted with the same task, might come to different conclusions without any one being justified in saying that either of them has lacked competence and reasonable care, still less integrity, in doing his work, Valuation is an art, not a science.”