Instead the optimum situation is where a keyword is sufficiently popular to achieve both good traffic, yet is close enough to the specific interest of the searcher that they may become a purchaser.

Ultimately, popularity of a search term is the more important of the two considerations, because if a lot of people see your offer even if it isn't exactly right for them, there is a chance that you will get a sale; whereas if you have the perfect offer, but no traffiic there's no possibility of selling.