What you describe is exactly what any self-respecting inventory management system will do. In fact, you are describing a low-end system. Any reasonably useful system will do more.
Essentially, the goal is to determine, for each product, the re-order level and the re-order quantity, based on the following factors:
Forecast sales (which in turn are based on past sales, growth trends, seasonal factors, and special circumstances);
Cost of ordering (the cost and effort of placing one order and receiving one delivery);
Payment terms (which affects how long your money is tied up in your inventory); this also depends on:
Price breaks (these are the discounts you refer to, where discounts are based on the quantity ordered);
Cost of being out of stock (this is mainly the cost of lost sales, but also includes the loss of customer goodwill; it depends partly on how likely it is that the customer will wait for the product to come into stock, or will purchase an alternative product).
There might be some others that I haven't thought of.
My point is that it would be almost impossible to manage a large inventory without an automatic way of determining the relevant figures. Many well-established and mature tools already exist - and have done for a long time. (I spent part of my career designing and programming this kind of system, and that was before personal computers were widely used; the market has become much more sophisticated since then.)
This is why I suggest that you don't waste time looking for customers who are not using such a system. Rather, you focus on looking for weaknesses and deficiencies in the existing systems, and then try to produce something better.