There is no complication. You can continue selling all over India from your Karnataka Office.
Whenever you plan to shift, you will have to get yourself registered under that State Laws where you are intending to shift.
Presently I want to start a business by selling online in the state of Karnataka but later I will be shifting to some other state and doing business from there. So if i register for VAT and CST here in Karnataka what will be the procedure, complications and amount required to do so.