Broadly speaking, this could be phrased as "in bulk", "bulk lot", "wholesale", and a few other terms. All of these relate to the quantity of an item being purchased (wholesale having a few other meanings around general discounts, such as "wholesale pricing"). Possible sentences include:
- I want to buy some apples in bulk.
- I want to buy apples wholesale.
- I want to buy a bulk lot of apples.
All of these are correct from a merchant's standpoint, at least in the western United States. However, as you notice, most are not a single word, and the one single word choice is ambiguous as to its precise meaning. But, generally speaking, phrases centered around the word "bulk" or "wholesale" are those used to communicate the idea of a discount because of the quantity of items being purchased.
"In Bulk" can be found defined in the Cambridge Dictionary (We buy a lot of our groceries in bulk (= in large quantities) to save money.), when searching Google for the definition of "in bulk" ((especially of goods) in large quantities, usually at a reduced price.
"buying tomatoes in bulk from a local farmer"), in the English Oxford Living Dictionaries, and in multiple other locations.
It is difficult to find a formal definition of "Bulk lot", but its usage as a term can be found in this article on buying wholesale or bulk lots on Ebay. Other examples of this usage can be found across the net.
"Wholesale" has a few different forms that imply the large lot, the low price, or both. As a noun, it implies both large quantities and low prices, i.e. [mass noun] The business of selling of goods in large quantities and at low prices, typically to be sold on by retailers at a profit.
As a verb, It signifies selling large quantities at low prices, i.e. Sell (goods) in large quantities at low prices, to be sold on at a profit - ‘imported clothing, which he now wholesales to 20 retail stores’
And as an adverb, it can imply solely large quantities or volumes (As a whole and in an indiscriminate way - ‘That definition appears to be borrowed wholesale from de Boinod's predecessor, Howard Rheingold.’), low pricing as if a lot was purchased in bulk (‘I'm not a distributor, but we do sell wholesale, and I know how big a pain in the neck small orders can be.’), or both large quantities and low prices (Being sold to retailers in large quantities and at low prices - ‘bottles from this region sell wholesale at about £72 a case’)
(All three of these definitions come from The Oxford Living Dictionaries definitions of Wholesale. There is also the adjectival form, such as Wholesale destruction)