Hmm, I've used the dry-erase trick on a Game Boy game once and it worked fine - never tried it on a GBA game, but the labels seem to be the same quality. Leaving it is probably the best option anyway. Let the buyer risk it. As for goo-gone, I've never used it for marker on a label, but that stuff will seep into the edges of the label if you get it too close, so for a GBA game it's not worth trying.
NFR games are cool, good luck with the sale.
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Every jumbled pile of person has a thinking part that wonders what the part that isn't thinking isn't thinking of