Why take the risk on buying fake,cheap and frankly nasty imports? - I am a games player as well as a seller and it annoys me to no end to see these people selling obviously cheap fakes.
These Fakes have major issues, They may not save correctly, the build quality is also dodgy using sub standard plastics. There are very easy ways to check even by the listing or asking simple questions.
If your bidding on an item check the location where its coming from if Honk Kong/Japan/Asia 7/10 times they are fake and in my view its not worth the hassle as this could bring extended delivery time or even hidden VAT.
Is the item as described as FLAT PACKED?? - If so avoid like the plague these are obvious fakes that have come from direct from the factory normally to a UK dispatch agent - Hence making the selling item a more favourable UK location.
However the above is merely a quick guide and sadly you may have already been sent/bought such an item. Heres some checks to make.
As mentioned a fake cartridge will be sent flat packed , The box is overly shiny and 5 mins in direct sunlight it will fade and crack. - There is no card-board insert ( these are part of 100% nintendo packing policy) The instructions will be very thin and contain basic instructions with no multi-language or safety guide. The Cartridge it's self will have very shiny label ( However take care as the US carts often have this) but the fake will generally be hand stuck and be NOT be central or straight. Also look for a small numbered impression on the label generally 3 numbers, These are stamped on US/UK carts or more importantly 100% original carts.Fake cartridges plastic build will be bad- thin plastic and possibly a flexing/chipping to the corners.
A GBA will allow you to play US/Jap carts so dont be afraid to ask the seller questions as I've bought US original carts and no issues with them. But why pay loads for a fake one - Yes it may work fine but they will go faulty and as mentioned might not save data.