Except that possessing a firearm, or intending to sell is not illegal*. Possessing a bag of weed is a crime and gives probable cause to search the rest of the car.
* With a few exceptions, though these need to be suspected before probable cause is applicable. The police could question the seller if she can legally possess the firearm or if she has intention to withhold selling to someone that can't legally purchase, perhaps other local restrictions, etc.