I bought this game years and years ago from eBay. I didn't have any problem with the game or anything but recently I saw a picture of a HM GBC 3 cartridge on the internet and it was transparent. My game is grey like most original Gameboy games. Was this game ever produced on a grey cart or did I buy a bootleg?
Wow... your cartridge doesn't even say Gameboy COLOR. It was the old GB style one xD Even though the label clearly says "only for Gameboy COLOR"
But your game actually works?
It's likely someone uploaded a pirated Very Bad Thing™ into a catridge, printed off a label(which if you look at the right of the suspect copy it isn't the same), and stuck it on.
My recommendation is to bring this to Ebay's attention. Assuming you still have the purchase information, either on your Ebay account or email address registered to it. You most likely won't be able to get your money back, and it is probably too late for Ebay to take action, and Ebay may not handle such situations directly. But at least it will make it possible for Ebay to keep an eye on the person who sold you the game. So that if they are still selling bootleg copies they will most likely get caught.