Most important goal of any commerce platform is the conversion rate. How many visitors turn into customers, what do they buy and from what device? Defining a new product, assumptions must be made on the various rates. When developing a new product that will replace an existing platform, minimal thresholds need to be set on the rate. Try to embed these in a lean startup approach (as described by Eric Ries).
Overall performance and software quality
There are also less visible indicators: overall performance of the platform and software quality. Low performance will result in a high churn rate (the rate of customers abandoning their basket and customer journey). Low quality will result in problems in the life cycle of the product, as new developments are harder to make and new features resulting in problems in existing code (regression). This is also called technical debt, as you can only bear so much debt in a product and the debt builds up gradually, usually under delivery pressure or quality problems that are not addressed. There are tools that can monitor performance and check the code against pre-defined coding standards. Automating testing will also help. With every release, tests are run easily.