The Federal statute restricting handgun sales to 21+ ONLY applies to FFL dealers, NOT to private sales.
Idaho does not prohibit possession or private sales of handguns to 18+ persons.
Your action was legal with no problems.
How did the dealer have any authority to enforce anything at a gun show? It wasn't his transaction. If he had a problem with it he should have reported it to the ATF, not messed with your sale. I would say you have a small claims court case against him, if you chose to pursue it.