I was unaware that it was more then just iTunes. I don't have the papers in front of me. I found one that was thrown away after typing out the code in my notepad.
You can check them using iTunes app or vudu. Depending on which it is. I check them every so often but I have had people that say they didnt use both parts of the code vudu/iTunes and turns out they do. So it's a trust thing. 95% of the time they only use the vudu one they are traded or sold but there is on occasion people that use both with out permission. So if the code doesn't work that would be the reason. But I check every time before I send codes. I also know which ones I've sold and which I haven't sold. There's a list.