VGC actually uses "polling based methods".
From
http://www.vgchartz.com/methodology.php
"All sales estimates on VGChartz are arrived at via a number of proprietrary and ever-developing methods:
- Polling end users to find out what games they are currently purchasing and playing
- Polling retail partners to find out what games and hardware they are selling
- Using statistical trend fitting and historical data for similar games
- Studying resell prices to determine consumer demand and inventory levels
- Consulting with publishers and manufacturers to find out how many units they are introducing into the channel
For all hardware and top 50 software, sales estimates will be arrived at from at least three of the distinct methods listed above to ensure accuracy. For software below the top 50, sales estimates may be arrived at from only one or two of the methods since less data (and time!) is available. For lifetime sales, particularly for games and hardware that have now stopped selling, data can be considered to be very accurate and will be checked against manufacturer shipment data. VGChartz holds no responsibility for the use of our weekly sales estimates – any business decisions made are made at your own risk."