If that is a battery acid stain on the outer box, I have a bad feeling about the actual console boxes and contents. Hopefully they're OK.
I bought old stocks of GBA and they had leaked. I had to throw away two boxes and all the contents minus the consoles themselves.
It sucks godzilla balls that Nintendo shipped batteries with GB/GBC/GBA, I've seen too many wasted boxes and contents which were otherwise new and unused. Makes me sick.