http://www.boe.ca.gov/cgi-bin/rates.cgi?LETTER=Y&LIST=COUNTY
Look at the link above for Yolo County California, and in particular the Davis, CA entry. Now lets consider 3 addresses:
617 2nd Street, Davis, CA 95616
1 Shield Avenue, Davis, CA 95616
1204 Landra Avenue, Davis, CA 95616
If you are Amazon.com what sales tax do you charge to send to each of these addresses? Well, the first is the address of one of Amazon.com's competitors, The Avid Reader. It charges 7.75% sending 7.25% to the state and 0.50% to the city of Davis. The Avid reader need only know about the tax laws in the location it has a store. The second location is in the city limits, but is the address for the residents halls at UC Davis. As you will note in the link, the University of California is exempt from local sales tax. The same book bought at the campus bookstore, or shipped to the residents halls should be charged 7.25% tax. The campus book store knows this but does an online retailer? Then there is the final address. It is in the same Zip Code as the city of Davis, and in fact uses the Davis post office, but it also should be charged 7.25% because it is actually just outside the city limits in unincorporated Yolo County.
This is a City listing at one point in time. On October 1 tax rate will change in some places and there are literally 1000s of different sales tax zones. A brick and mortar store need only know about one but an online retailer would have to keep up with all of them, in every state!. As you can see in the example above, you cannot go by city or Zip Code. Each address needs to be tagged with its correct zone.
As anyone can see, to compare a 7-11 to a major on-line retailer is just absurd.