A brand new sealed copy of Ninja Five-O for $4.95.

Ninja Five-O is a hidden gem for the GBA, offering an experience that offers the gameplay from Mega Man and Castlevania, unique levels, stunning boss battles, and a grappling hook. It is also a one of the rarest games for the GBA. The last time I checked ebay, a sealed copy was $500, and the cart alone was $45.
So why do I hate remembering this memory, you ask? (Yes, my copy of Ninja Five-O is still sealed and will stay sealed)
Well, right next to the Ninja Five-O, I noticed 2 new boxed games I had never heard of before, so I didn't buy them.
Those two games were Golden Sun and Golden Sun: The Lost Age.
