like most crimes it really comes down to this:
A-most experienced (i.e. those with the money to make the con worthwhile) merchants will likely have seen it or something like it before.
B-the crime isn't nearly as clever as those attempting it think it is.
C-could you get away with it under the right conditions (a crowded bazaar on a busy day) sure. but not likely very many times before people catch on.
D-And the whole 5'/1 minute limit on a pact blade being out of your possession hurts the con a lot. IN combat, 1 minute is a long time. OUTSIDE of combat, even as a head start to escape, 1 minute is next to nothing.

basically the entire con comes down to being an a-hole for either very little profit and/or very high risk. Its going to fail the career con-artist's litmus test of drive/risk/reward.