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Where did you get the serial number? If you got it from OS X (through System Information or About this Mac), you can be sure you have an original iMac. The serial number is attached to the logic board of your iMac, so you can be sure you have got a 27-inch iMac.
Sometimes resellers (and even Apple) do not sell Macs in the original packaging. This is often done with second-hand or refurbished Macs
Thanks for your reply. I got the serial number from the about imac section and I verified on the apple site and it looks like a genuine one as apple identified the version number. However, I enquired with the seller and he says he posted as he got from Apple. I still did not get it why I did not get it in the original Apple imac box as the seller insists its a brand new one. Any ways, thanks for your reply. I really appreciate.