Sales Tax is History. The new tax is called VAT (value added tax). unless you live in one of the few states which have not adopted VAT.
VAT is applicable to every manufacturer or trader who annual sales value is over Rs.5,00,000/-.
Selling offline / online is not a distinction. If you feel that you will be doing a business of over Rs.5,00,000/- in an accounting year (April to March) you will need to apply for a VAT no.
Appicable of CST (central sales tax) is a debateable issue but I know some sellers pay 100% CST and other sellers who only play local VAT and no CST.
Infact, if you are thinking of doing an online business (and not a few one off transactions) you should consult a good tax consultant before emarking upon such activity.