Once you've made the decision that you are going to start selling on Amazon, we believe that you should start tracking products using a tool like ZonGuru as soon as possible. The reason we say this is because if you use a software like ZonGuru to track sales, those sales numbers are collected by tracking actual inventory levels over time. In other words, the longer you have been tracking a product BEFORE you make a decision to enter that market, the more data you have to base that decision on.
There are other Amazon software products in the market that use algorithms to "estimate" Amazon sales. The upside of these types of softwares is that you can get your data instantly, the downside is that the numbers you are seeing are based on an algorithm - not actual historical sales. They're projections rather than facts. While these are pretty accurate for the most part, our data has shown that inventory tracking over time is the most accurate method.
For these reasons, we recommend that you start tracking Amazon products as soon as possible.