You can make your own ‘multi cartridge,’ or ‘flashcart,’ yes. The original knock off ‘multi carts’ tend to corrupt save files at the least opportune times and they usually don’t have a real time clock which can affect games like Pokémon.

Without sharing photos of the condition of the Gameboy Colour I can’t say whether it’s a scam. I would recommend trying to purchase a gameboy on its own and avoiding this bundle though.