I purchased TurboTax Home And Business 2017 from greentaxexpert.com, which I now realize appears not to be an authorized reseller. Is there any way to verify that the download has not been modified or infected by them (i.e. an SHA hash of the official download).
Sorry, they don't release their hash values, for various reasons. You can try and return it, or dispute the charge with your bank, or take the chance.