"The best-selling video games" refers to the actual games and which ones are the most popular.
"The best video games sales" is rather awkward phrasing without further context; but the main difference is that the games themselves are not the subject of the phrase, but rather the amount of sales. So we might say "The store with the best video games sales in New York City is Blockbuster Games. They sold 4% more merchandise than any other games store in the city." Or it might refer to a discount on the price of video games: "Blockbuster Games has the best video games sales! I bought Super Mario Brothers Versus Godzilla for only $2.99!"
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