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eBay that isn't hundreds of dollars (and some that do cost hundreds of dollars) is actually a cheap flash chip and a CLPD CPLD to simulate the mapper wired to a bootleg PCB. These bootleg carts can be bought blank on aliexpress for peanuts and many can be reflashed with a dumper/flasher device like [https://bennvenn.myshopify.com/products/usb-gb-c-cart-dumper-the-joey-jr Joey Jr.] These bootlegs almost certainly use 3.3V chips wired to the 5V cart bus. This could shorten the life of the cartridge and/or the Game Boy.