I can see a pattern of discrepancies when comparing the prices from Pricecharting to the prices of Gamevaluenow. When it comes to more valuable games, GVN's prices are often higher, sometimes by a significant margin.
One significant discrepancy involves NBA Jam: Tournament Edition, SEALED/NEW; Pricecharting has it at $63.07 while Gamevaluenow has it at over $248!
The discrepancy isn't usually as severe, but it's a glaring example.