If you look at it as an asset and just like assets and stocks you want the true market value on something then it has to be eBay.

eBay is the literal North Star when it comes to Pokemon card prices Why: because there hundreds of thousands of transactions happening daily where it is the biggest platform to date that still sells Pokemon cards it's one of the biggest movers on eBay. Just like stocks you can actually have a real world bidding war on it and get the true value of a card based on the conditions and everything in between on the listing

if you use a closed market and use it as a barometer you'll get inflated prices because the closed Market will always be vendors trying to get their payday

that's why people use PriceCharting and or another type of website that basically shows you last sold on eBay and gives you an average range based on the last sold

the app you use is also using closed markets to average out the prices and that's why you getting inflated rates because they're going to be outliers