States are very territorial in regard to attempting to collect tax from businesses having operations within their state. California is one of the more aggressive states in this regard. Under California law, if you have agents making sales within the state you have nexis, allowing them to subject your California sales to income taxes. Your sales reps are considered your agents if they are employees or independent contractors. These sales are also subject to California sales tax. You can receive a credit on for state taxes paid to CA on your state return. www.ftb.ca.gov/businesses/New_Rules_for_Doing_Business_in_California.shtml

You will need to register to do business in California. http://business.ca.gov/StartaBusiness/RegisteringaBusiness.aspx