I can tell you from experience that a good HALF of players and their characters will be 100% clueless.Wow, wow, wow,
The trouble for the trader is, if the character finds out (and they normally will) that they've been duped, they'd likely seek retribution.
If your not a "fan" of the players, and just tell them somehow, they will likely not figure it out.
But then too....let them find out. The trader guy that makes/has made cursed items has plenty of defenses.