That is where the batteries are always packed in within the box, so I assume it's from the battery acid over time. Game boy is sealed inside the bag, so there should be no wear on the Gameboy itself on the inside.
Box itself outside looks NM and is gorgeous! Attention: This is a high end collectors item, and an official authentic sealed Gameboy color, limited Pokemon edition!
Please do see the box and condition for yourself. I CAN NOT TAKE ANY CHANCES WITH ITEMS OF THIS CALIBER.
Please ask for any questions! The item "Nintendo GameBoy Game Boy Color Pokemon Limited Edition New Factory Sealed" is in sale since Thursday, April 16, 2020. This item is in the category "Video Games & Consoles\Video Game Consoles". The seller is "bills_tcg_cardshop" and is located in Denver, Colorado.This item can be shipped to United States.