predicting number of customers based on long/lat coordinates
The answer relies on the question: are sales related to location?

There might also be some linear relation between location and sales in the sense that "the further you go away from city centre, the less books people read", which is definitely worth checking by simply running a correlation analysis between longitude,latitude and sales and see it in a matter of seconds using either Matlab or Python (Pandas).

Another thing that you can do, since you have two inputs, you can plot the sales for each location (longitute,latitude) on a 2D colormap (heatmap), where color intensity shows number of sales. Let us assume that light and dark colors correspond to low and high sales respectively. If you see that similar color intensity is gathered in same regions of the map, it means that location actually affects the sales. On the other hand, if you see randomly colored points in a small area, this means that sales are irrelevant to location.