Unless you made the installment sale to a related person (see below), you are not required to report the buyer's tax id number (SSN or EIN) on the actual Form 6252: Installment Sale Income. The IRS asks for the Buyer's SSN on Schedule B, however, its inclusion or exclusion will not affect your tax return (it may, however, affect your ability to e-file).
If the Buyer was a related party, you will need to get the SSN whether you e-file or mail the return.
If the Buyer is not a related party and you want to e-file, you will have to enter an SSN for the buyer, wherever the program asks for it.
You can enter your own SSN if you like; it doesn't affect your tax calculation, You can enter a "dummy SSN" (e.g. 305-44-0001, a dummy number we use for training). I personally would just use my own SSN. If you don't feel comfortable with this approach, then you will have to file by mail without the Buyer SSN (write "Not Available").
Related persons include, for example, you and a member of your family (spouse, brother, sister, parent, child, etc.), you and a corporation in which you have more than 50% ownership, you and a partnership in which you directly or indirectly own more than a 50% interest of the capital or profits, and two partnerships in which you directly or indirectly own more than 50% of the capital interests or profits.
And on what form/interview are you getting the error for missing Buyer's SSN?
Form 6252 Installment Sale AND Schedule B Seller - Financed Mortgage Interest Statement
Unless you made the installment sale to a related person (see below), you are not required to report the buyer's tax id number (SSN or EIN) on the actual Form 6252: Installment Sale Income. The IRS asks for the Buyer's SSN on Schedule B, however, its inclusion or exclusion will not affect your tax return (it may, however, affect your ability to e-file).
If the Buyer was a related party, you will need to get the SSN whether you e-file or mail the return.
If the Buyer is not a related party and you want to e-file, you will have to enter an SSN for the buyer, wherever the program asks for it.
You can enter your own SSN if you like; it doesn't affect your tax calculation, You can enter a "dummy SSN" (e.g. 305-44-0001, a dummy number we use for training). I personally would just use my own SSN. If you don't feel comfortable with this approach, then you will have to file by mail without the Buyer SSN (write "Not Available").
Related persons include, for example, you and a member of your family (spouse, brother, sister, parent, child, etc.), you and a corporation in which you have more than 50% ownership, you and a partnership in which you directly or indirectly own more than a 50% interest of the capital or profits, and two partnerships in which you directly or indirectly own more than 50% of the capital interests or profits.