If you bought an actual N-card(and not some other Slot-1 based card) then you definitely need a gba sized linker to write to it- if it didn't come with one, you'd better contact the guy you bought it from.
Multiplayer is only supported with the latest firmware; and if a game doesn't boot then you've either got a bad rom or you may have accidentally switched the save type(this can be changed by preeing left or right while holding select).
Last, but not least, how do you accidentally buy something on eBay? I can't even find backup devices there, except the time a guy sold a used DS Lite and an R4 with 20 games on it for over $300 USD.
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