How to enter 1099-K

It depends what the 1099-K is reporting.

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 and select the box for “This amount in box 1a is too high or includes some personal transactions” in order to enter any personal items such as birthday gifts.
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, even if only one
Choose “All items were sold at a loss or had no gain” If ALL the items were sold and you did not realize a gain on any single item, (they ALL sold for less than or only for as much as you paid to purchase the item in the first place) you would not need to enter any sales and you would be done reporting Form 1099-K at this point.

If you need to continue-

Enter the amount of the 1099-K that represents the cash you got for selling items at cost or under. 
So, if the 1099-K reports $1,000 and the only item you sold at a profit was an item that sold for $50, you would enter 950.
$950.00 on that 1099-K does not need to be addressed. 
Also, remember that this is NOT saying you MADE $50, only that someone paid $50. The profit would be less because you need to subtract what you paid to buy it. 
CONTINUE

Finish the interview and select done on the “Your 1099-K summary” screen, 
 

YOU ARE NOT DONE if you made the first choice that only SOME items sold at a loss,
that would mean that at least one item sold at a gain, for a profit. 
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 deducted previously, 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