Business & farm

If you sell items online as a business, enter on Schedule C
If you sell personal items online similar to a garage sale enter following these steps:
In TurboTax enter the 1099-K under:
Income
Other Common Income
Income from Form 1099-K     START
On the “Did you get a 1099-K?” screen, select “Yes”
On the “Which type of income is your 1099-K for?” screen, select “Personal item sales”     CONTINUE
Enter the 1099-K as it was reported 

CONTINUE

On the next “Personal Item Sales” screen, make the correct selection. 
Choose “I sold some items at a loss or had no gain” if you made a profit on any individual sale

CONTINUE

Finish the interview and select done on the “Your 1099-K summary” screen, 
YOU ARE NOT DONE 

Now you need to report the sale, go to 
Income
Investment Income
Stocks, Cryptocurrency, Mutual Funds, Bonds, Other    click UPDATE
The next screen should show “Personal items sales (1099-K)” 
Click REVIEW to open 
The next screen has an input screen for the Personal items you sold at a
profit/gain.
Enter one sale at a time.
Report when purchased, when sold, how much you paid to buy it and how much they paid to buy it from you

The next screen asks for selling fees, but those are USUALLY deducted before they pay you, so would not be included on Form 1099-K
If they were not, enter here

Once done, you should see the amount of proceeds you needed to address ($50) and the profit you made from that (those) sales. 
The profit is Capital Gain listed on your 1040 line 7