I don't know the specifics of your purchase.  But if the registration key has already been used, then the 'second hand' vendor sold you a defective game:  (no valid key = zero value of item).  If above is true, then vendor should refund all you money including any shipping charges, and tax.  It the vendor's 'bad.' If vendor won't - report them.  Be sure to find out the physical location of vendor and then report them to Better Business Bureau.  I suspect there are also 'on line' places that track the honesty and integrity of vendors.  report there as well.  If the 'someone' you got the game from is not a vendor, but an acquaintance, (or Craigs list) etc.  good luck.  (really!)