How do I report a sale of private company stock that issued a 1099-B for the full proceeds, but will make installment payments over 5 years? Do I ignore the 1099-B and just report installment sale via Form 6252?

You would have to report the sale on form 6252 for installment sales. If the stock had been reported on an exchange as opposed to being private stock, you would not be able to use the installment method to report the income from the sale. You should also enter the Form 1099-B in the investment income section of TurboTax but list the cost basis equal to the sale amount. This will prevent the IRS from adding that income to your return when they match up the 1099 form income reported to them with your investment sales on schedule D of your tax return.

You can use the search icon on your menu bar in TurboTax and enter "Installment Sales" and you will see a "jump to" icon to find the installment sales income entry screen in TurboTax.

Thank you.  I had already completed Form 6252, but didn't know what to do to address the 1099-B, as it was adding the full gain to current year.   I did note that Box 6 did not have checked either the "Net Proceeds" or  "Gross Proceeds" and Box 12 was also not checked, so was thinking maybe the company didn't actually report the 1099-B.  Thanks for your help!

It is possible the sale was not reported to the IRS, but it won't do any harm to make the $0 income Form 1099-B entry just to be sure.

See Form 6252 Lines 26 and 47.

Then see Schedule D Lines 4 and 11.

TurboTax should provide the correct reporting of your Installment Sale Income

Thank you for your response.  Form 6252 is already reporting the correct income on Schedule D.  My issue is that Schedule D is also reporting the full gain from the 1099-B form.  So, if I enter the 1099-B form as is, Schedule D shows the gain from 1099-B AND Form 6252.  I just want the gain from Form 6252.  

Thank you again.  I will give this a try.  It just makes me nervous adjusting the amounts that are reported, but I don't see any better option.  Appreciate your help!

What is the Box Category on 1099-B ?

For non-covered securities ( BOX B or E) showing zero (or wrong) basis, enter the correct basis yourself.
Use code "B" and put -0- in col (g).
For Box A or D, leave basis as reported, use Code "B" and put an adjustment in col(g).

Alternatively, in your situation,  instead of Code "B" you might  use Code "O" (other).

Code "B" is more common.

@Legtag1990 

Thank you!  I actually used form 8949 and used code B last night.  I think this will work.  

Thanks to both fanfare and ThomasM125 for your help!

If its private stock how do you report 1099-b. Isnt that for public stock sales? I didnt get any paperwork for sale of private stock it was private lol