Do i need to report a 1099k from paypal even if it is under the $20,000 per year and i was only selling personal stuff and i didnt make money from it?

Normally, you would not report the sale of your personal items because you sold them for less than you paid for them. 

One option is to leave it off and if the IRS questions it, then show them your documentation.  Print out or screen shot something from Paypal showing what you sold was used personal items.

But, since it was reported on a 1099-K and the IRS receives a copy, I would report it this way.  It zeros out, so it doesn't affect your taxes, but it acknowledges that you included the 1099-K on your return:

Enter it under Wages & Income tab at the top: 

1. On Your 2017 Income Summary, scroll down click on Start next to Miscellaneous Income, 
2. Then scroll down and choose Other reportable income
3. Answer Yes to "Any Other Taxable Income?" 
4. Under Description put "1099-K Personal Property Sales "
5. Enter the amount from the 1099-K in the next box, click continue, 
6. Click on "add another income item", 
7. Under Description this time put "1099-K Cost of Personal Property"
8. Enter the same amount but as a NEGATIVE number.
9. On the Other Miscellaneous Income Summary make sure you see the 2 items, one positive and one negative and Total is $0.

The IRS will not allow you to deduct a loss on personal items that you sell.

can i still follow this process if i already received my federal return check.  i received this 1099k after i did my taxes...

No, you don't want to do an amended tax return, unless it changes the amount of your refund or amount you owed. In other words, the IRS doesn't want to process an amended return that results in no change to taxable income.  Just keep you records showing you sold personal items and if the IRS ever asks, you can explain why you don't owe taxes on that income. Keep the documentations for 3 years at least.

Great, Thank You!

How do I report a referral rebate reward bonus payment (which is in the 1099-K) on my tax return?