Sorry to say that your theory is incorrect. All manufacturers require proof of purchase in order to fulfill your warranty request, meaning you need to provide a copy of the original sales receipt (which states the date of sale). For instance with Sony, they ALWAYS require a receipt with PS3 warranty service, otherwise the warranty period cannot be determined and the customer must pay for the repair, or it is sent back to them minus the cost of shipping.
With the 360 specifically, when Microsoft bills the retailer for a shipment for example, they likely keep track of the SKU's/Serials in the shipment which gives them a ballpark of the warranty start date (these systems don't usually stay on store shelves for more than a month for example, as they are restocked weekly). I've never personally sent a 360 in for warranty service (only a YLOD PS3), but I'm pretty sure Microsoft also used to require the original receipt for proof of purchase for warranty returns. For the last 2-3 years however, as they extended the 1-year warranty to 3-years for the GPU defects (RROD), they may not have required a receipt as they would know for sure if it's been under 3 years due to the manufacturing date (but I'm not 100% sure on that). Seems logical that they would still require a receipt though, just like any product you would return or send in for warranty (even non-video game related)

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I worked as a merchandiser and cashier at Futureshop (in Canada) for about 2 years before moving to sales, and I can tell you with 100% certainty that we only scan the barcode for sales and inventory purposes, so there is no such thing as a individual company/reseller requiring a seperate database to collect serial numbers upon being sold (ie. Microsoft)

. I can't imagine any retail chain doing this in the US either, or anywhere else in the world for that matter, unless they were owned directly by the specific manufacturer in question (like an Apple store selling Apple products).
I hope that makes sense. I appologize for this being completely off topic. Now to start gutting the slim 360 I just brought home this afternoon

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MS do ask for proof of purchase date if some one tries to get a repair under 3 year warranty when there records show it out of that period.
I just asked my sisters husband who works at head office for Game group he said some Game stores store the purchase record for 3months maximum it's store managers choice but they have to at least store it for 30days but head office has a record of the sale for between 6-12months depending on the item this is a customers purchase record they do not log serial numbers as stated above scanning barcode is for stock records only unless you take out an insurance policy.