Nintendo Eshop code already used?

I bought a download code on eBay today, but the shop says "The code has already been used". I don't know if I was being ripped off or if it was a bug, because the first time I entered the code I got a timeout as an error message and was kicked out of the shop after I activated the code. Well and when I restart the shop it says the code has been used and it is not listed under my games, so downloading again does not work…

Probably someone gave you a used code on eBay.