It needs to be more concise. How about something like:
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Video game sales numbers aren't freely available; companies must buy them. VGChartz makes their numbers available freely, collects their own data, and can be easily compared to professional tracking firms. In fact, they post comparisons as the data becomes available. Their accuracies are typically very good, and where they diverge from the professional tracking companies, they update their data.
VGChartz gathers data in the same way professional firms do -- they collect the data from retail and online stores. VGChartz has 3-5% coverage, which isn't as high as most professional firms, but it provides its numbers for free. While the VGChartz team is not 100% accurate, and no tracking firm is, it has proven through time that its accuracy is quite high.
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