As far as I know, Turbotax will not help you authenticate the software. There are ways, using cryptographic hash values, to verify that two files are identical. However, there are hundreds of different Turbotax installers (10 standard versions with a new installer released for every program update, which are roughly weekly) and even if Turbotax would give you the hash value for the particular installer and version you have, that would only serve to help the eBay seller cheat them of their profits. You could call support and ask but I don't think they will tell you anything other than to uninstall and re-buy from a valid reseller.
