Just to be clear, you use Gameguardian to perform a negative exchange, where you can receive the purchasable item for free
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* - For nopaew, you pay it for free, meaning your currency doesn't get below 0.
But for burning books, your traded book can go into the negatives. This means that if you traded 999 Murasame starchart for x9990 Katana Luring Shadow, your Murasame starchart will be -999 in your inventory (assuming you had 0 prior). And if you do get a Tsukiha drop, said starchart will be -998.
While technically you can stellar awaken a unit by buying the Allcosmos for literally free, your inventory for some reason will just list the starchart in the negatives, which aesthetically looks bad.
If you don't mind the listed negative values in your inventory, this is probably a faster way to get l/s points than completing content and using external websites to track where l/s points drop in a chest.