After I filed, I received a 1099-B showing a loss of $129. Is this worth amending?

Regardless of whether you had a gain, loss, or broke even, you should report all of your 1099-B forms on your tax return.  

The IRS may reject your return due to the missing form.  If so, simply log back in and enter it.

If not, you should amend your return, but not yet.  Allow your initial return to be processed first.  

Allow at least 16 weeks for the IRS to process your amended return.

TurboTax Online:

1. Sign in to TurboTax
2. On the home screen, scroll down to Your tax returns & documents and select the year you want to amend
3. Select Amend (change) return, then Amend using TurboTax Online
   • If you're told to amend your return via TurboTax Desktop, follow the steps for TurboTax CD/Download below instead
4. On the screen OK, let's get a kickstart on your amended return, select the reason(s) you're amending and Continue
5. When you reach Here's the info for your amended federal return, select Start next to the info you need to change
6. Continue through the screens, make the changes you need to make, and carefully answer the remaining questions to finish amending your return