I was shocked to read these guides as i did not realise this went on however i did buy pokemon sapphire from ebay and when i recieved it it was american and it works perfectly fine on my daughters ds just like an normal one would.
So i bought another pokemon gba game from ebay also it was another american game. When my daughter started it it came up with the message ' save file has been removed due to corruption or damage' which meant she could play it fine and apparently save it fine. She then turned her ds off. When she tried to resume her game the only option she had was options or new game
Its terrible people are doing this. It is so easy to check if the game is fake so this means the sellers are passing them on knowingly.
Having compared the cartridges i have found that if you look in the opening at the bottom of the cartridge it should be 'filled in' roughly half way down with no gaps. There should also be an official nintendo code printed along in the inside.
On the fake game i had there was no code and it had a chunk missing from the inside which i assume was the save file deleted