Building the powerstore by hand, even with our tools, takes a lot of knowledge of php, mySQL, and mastery of our product line. An assumption that you could create any possible solution without any knowledge of the underlying code or products once you purchase the super suite is not accurate, so I apologize you were under that assumption.
Discounts can be easily added with eCart to the powerstore or a store you build on your own. There is existing eCart documentation and tutorials on how to add discounts. Inventory control is a little trickier, since, at least in powerstore, some techniques require writing complex SQL to deduct inventory and check inventory levels as items are sold. You need to understand your database and how to reference data from it in order to be successful and unfortunately we don't have a tutorial on this specific subject available at this time, but it is implemented in the powerstore solution.