Both are fine. But they have a couple of differences of nuance.
To market something at someone or some market segment can convey the sense of the marketeer 'aiming' at that target from a distance.
Where one is marketing something to someone, the sense more often conjured up is one of the marketer 'bringing' or 'presenting' the relevant product to a specific audience.
Using 'at' sounds a little more impersonal (and perhaps more aggressive), while choosing the preposition 'to' lends your sentence a possible nuance of active close-up contact with the people for whom the product is intended.
You would be more likely to market (say) a cigarette brand at a particular target market, while a product intended to genuinely solve potential customers' problems will tend to be marketed to such customers.
Think also of the difference between throwing a ball to someone and throwing a ball at someone.
More generally, one tends to go to people (personally) and aim at targets (or people) impersonally and at a distance.