As far as I know, all DS games besides Nintendo's are shrink-wrapped. So, if you have access to a shrink-wrapping machine, you can always seal it back up without anyone knowing that it has ever been opened.
I first came to this conclusion when I picked up Castlevania: DoS. I picked it up the first day it came out, saw the wrapping, and thought that it was odd. Then, when I turned on the game, the first message I got was something like, "The save data is corrupt. Would you like to delete it?" or something like that. Thinking that it was a faulty game, I took the game back the next day for a new copy. That's when I saw that all DS games besides Nintendo's have cheap wrapping.
Back on topic....
Personally, I don't care if someone opened a game, that they bought, before giving it to me.