Get your taxes done using TurboTax

First enter Form 1099-K

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 

NOTE: ONLY select the box for “This amount in box 1a is too high or includes some personal transactions” if it shows things like birthday gifts you received or when someone sent you money to split a dinner cost.  
CONTINUE

Next screen make a selection-

IF ANY ITEM SOLD AT A GAIN- select  “only some items had no gain”, and enter the amount that represent the items NOT sold for gain.

In other words, if the 1099-K shows 1,000 "Proceeds" and 900 of that was stuff you sold at a loss or broke-even, enter 900 here. 
In this example, you would need to "account" for 100 "proceeds". That means 100 was sent for something you sold and you sold it for more than what you paid for it.
That does not mean you MADE 100, just that they sent you 100. You must record what you sold, what it cost you, and the profit you made. You do that later in the "Investment" section
CONTINUE

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

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 what you sold, when you sold it, when you purchased it
Enter how much you sold it for, and how much it cost you to buy it.

The difference between what it cost you, and what you got as "Proceeds" is the taxable profit (Capital Gains)