What is the source of this view from the classical Fiqh works
"You will use the price a willing buyer will pay a willing seller to purchase the entire goods at once"
and the logic behind it?
This wouldn't be the wholesale price, but would instead be much less than the wholesale price.
A person could have a shop stocked full of goods, which he would expect to retail at R500 000.
If each item had to be calculated at its individual wholesale price, the value of the stock would probably be around R350 000.
However if he has to look for someone to purchase all his stock in one go, he would probably struggle to find someone to even pay him R250 000.
Similarly is the case with a manufacturer. If he attempts to sell off all his stock to one person, he would have to sell it for at least half the price he normally sells it to wholesalers for.
Then what price would a wholesaler use?
The price he buys it for or his selling price, as the wholesalers selling price is what is termed the "wholesale price" in the market, yet no willing buyer would ever pay that price if taking the entire stock.