The mission of pricecharting.com is to estimate "market price" for video games, which typically means what price a game sells for.
Reading the crawled non-auction listings for eBay, the site seems to be showing the "Buy it Now" price. But, eBay has a feature for buyers to put in offers on these "Buy it Now" only listings, and if the seller accepts one of these offers, the game typically sells lower than the "Buy it Now" price. And eBay doesn't appear to make the actual sales data public on these type of purchases. This means estimates on pricecharting.com could get a little inflated, depending on how much influence these "Buy it Now" eBay listings have.
You can publicly tell if a listing is accepting offers via the "Make Offer" button on the site. I think it would be useful to know that information from the crawl, and either use it to influence the estimate or simply annotate it on the web site.
Though, maybe you are already doing this in your estimation, and in that case, it could be noted in your methodology. :)
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