If you would have to create a data base for geomarketing applications yourself, what kind of datasets would you purchase/collect? The first goal would be to find new store locations. Another would be to assess the efficiency of advertising efforts in different areas.
Demographics are probably the most important of your data sets in geomarketing. If you are planning on using the US Census/ACS (American Community Survey) be sure to research the proper methodology for using the rolling average that is the ACS. We no longer have a true snapshot now that the census long form is gone. Additionally, there are demographic data providers that create estimates on an annual, bi-annual, and quarterly basis if the Census data does not meet your needs.
Aerial photography. NAIP is widely available at decent resolutions for free.