The SSN is required only if the buyer is using the home as a personal residence.  At least that's what the IRS insructions say, but they have no way of enforcing that because you don't need an SSN to buy a home in the United States.  It's just one of those things on the IRS wish list, that they try to make look mandatory when it isn't, and they don't have the funds to match Schedule A deductions and Schedule B income anyway.