For a sale to take place on the hypothetical open market we assume that the sale is not sudden or forced and that there is
In some cases there may be a special buyer who is prepared to pay substantially more for an asset than anyone else. In general you should take account of the special buyer in considering what the market value will be but the market value will be less than what the special buyer is prepared to pay.
For inheritance tax purposes the sale is supposed to take place at the time of the transfer. But in some cases, such as the sale of land, buildings or household goods you may accept an actual sale within a reasonable time of the death or transfer as representing the open market value at the time of death.