Finding cheapest combination of items with conditions on the selection

Question

Lets say that I have 3 sellers of a particular item. Each seller has different amounts of this items stored. The also have a different price for the item.

Answer 1

This is a Bounded Knapsack problem. Where you want to optimize(minimize) the cost with the constraints of price and quantity.

Read about 0-1 KnapSack problem here. Where you have only 1 quantity for given supplier.

Read how to extend the 0-1 KnapSack problem for given quantity ( called Bounded Knapsack ) here

A more detailed discussion of Bounded KnapSack is here

These all will be sufficient to come up with an algorithm which requires a bit of tweaking ( i.g. adding 5$ when the quantity is below some given quantity )