Help! Cartridge Not Working

The eBay seller claimed to have tested the cartridge on a Gameboy Color, and even refunded the money to my account when I said I was having trouble playing it on my GBA SP. The game stays on a blank screen after the “GAME BOY” intro sequence finishes, sometimes being black or white on different startups. I checked the legitimacy of the copy and it seems legit to me which makes me more confused. If anyone has any ideas on how to fix this, please let me know!