Yes, also please read the rules before discussion regarding this topic here. You can get your money back for the dodgy product and buy a proper version.
When you buy Windows what you are really paying for is the Product Key. Only 1% of the price is for the DVD.
If the seller already registered the Product Key then you have been ripped off. The seller probably made a copy of the DVD and sold you the original one. He can reinstall Windows 7 any time on his computer using his DVD copy.
Try your hardest to send the disc and product key back to the seller and then buy Windows 7 from a trustworthy source. Newegg should be fine (follow voyagerfan's link).