I think this won't be a matter if the kpop fandom didn't make this as an achievement.
Or if they still want to brag about this, then I do agree with you and the criteria should be changed from:
"The most sales"
Into
"The most generated revenues"
Because like you said, multi version and cheap price could indeed inflate the sales count.
And in the end sales is sales..
But if you changed it to revenue counts, it'll shows a different level. Which is another perfect thing for kpop fandom to brag about.
Like instead of "X had 1M album sales" would have a different impact from "X had generated 1M usd revenue from their album sales"