contact the manufacturer(customer care/Service center) for confirming serial number.
always take a proper bill with tax paid; Do not get under the sales person's influence to get the bill later if you do not know him personally.
most of the pc peripheral is bit complex to be duplicated and sold with higher margin than the manufacturer itself.
also.. while buying retail boxes you can check the box for distributor's/(importer's) info... make sure it is based in india to ensure local replacement.
most parts to be duplicated are flash drives, external HDD casing etc.
Also... apart from duplicate stuff; most retailers use a cheap clone alternative.
For example, a retailer may fool a customer into buy K7 antivirus if customer ask for 'kaspersky or something'.
I have witnessed people selling inteX lcd, speakers, power supply after confirming with customer whether they need an inteL. so beware of such tactics...i know this sounds too silly but its not so silly when the customer is enthusiastic about his new PC and the euphoria can get you in the wrong deal. so just keep your ears open to clearly listen what they say and clear all sorts of doubts you may have during the purchase.