If I got it right, then your ssample size would be 700, which is the number of drills purchased, since you are not interested in the consumer but in the purchase. Beacuses of this, your observational unit is not the person, but the purchase itself in this case, so the sample size is the total number of purchases. Just consider that the proportion was calculated as ( Purchases in Store A / n), where n is the total number of drills bought. With that consideration in mind you shouldn't have problems with the analysis. Notice that you can also analyze other variables measured in people, just remember to identify the unit you are measuring in each occasion.