I can't comment directly on your situation, since my knowledge of AKs is limited. But in general, NEVER trust a dealer when he says something is pre-ban. That is an over-used and abused term. It's usually a gimmick on the part of the dealer to con you into paying more for something. And he's usually wrong. To my knowledge, there is not any legal recourse if you're busted doing something wrong with a post-ban firearm that you thought was a pre-ban. The onus is on YOU to comply with the laws. If you can make a good argument against the dealer for lying, he may get a slap on the wrist. But you'll be the one going to jail for the violation.
It almost seems that the laws for imported semi-autos are written to specifically create pitfalls for us. Many people try to push the limits with some hazy interpretation of the law. The BATF is not so generous with their interpretations. When in doubt, play it safe!
FYI, being a pre-ban weapon does not automatically allow you to create a semi-automatic assault weapon. It had to be a semi-automatic assault weapon before the ban went into effect. Altering it after the ban is illegal, even on a pre-ban firearm. That's the kicker that most people don't realize, and that's why dealers make so much money on the "pre-ban" myth.