Alan,
If the dealer was a known good person and dealer, I'd probably have brought it to their attention. I hope you don't feel bad about it because the dealer should know what they're doing.
If the dealer was a jerk, then definitely, no way I'd tell them until after the cards were purchased.
This would be a good poll question, but I'm going to guess most people would indicate the dealer should have known what they're selling.
Just curious, about how long ago was the purchase made and what was the value for those cards at that time?