@timbyto seems to be isolated to the game since it gives you issues on other consoles as well, which will rule out the card reader in your 2DS. I suggest you once more clean the contacts. Open the cartridge uop [guide|143740] and clean the contacts with a good pencil eraser. Sometimes a bit of metal polish and a soft rag will help as well. Be considerate of the fact that this removes material so you want to use it sparingly. Then try again.
You are right of course. It is a used game so you really do not know the history of how it has been "treated" etc. Ultimately that is the only thing you can try and do.