Anything less then $600, doesn't need to showing up in your tax return ...
I took a HR block class, the instructor said so.
They are after big fish, not tiny tiny $6.
You need to ask for a refund. They were wrong.
There are no limits for capital gains reporting in the US. Every $1 gain should be reported.
The $600 common misconception is the limit at which a business needs to issue a 1099 form. Nothing to do with gains.
Theoretically, you are right, however, instructor gave example like bank interest, go figure ...