Online seller

i am an online seller. i stay in silvassa. my purchase is weak here in local. i want to purchase from gujarat. but i am not clear with tax! the person who gives me goods from gujarat gets the product for which vat is already paid. and to sell from gujarat to silvassa he needs to add cst. so the product cost becomes higher and what furher i need to pay tax or will get any deduction. plzzzzzz explain!