I sold some personal items on ebay and received a 1099-k. I am in Mass, where the limit is $600 - I made $606. Where do I include this income on my tax return?

Yes, you must include all your income, whether it is reported on a form or not.

 You have two choices of how to enter the income, depending whether or not you consider this a business.

If you enter the income on Schedule C, you may take expenses of sale as well and include your cost.

However, since this is probably not a business, you can enter it as other income. Follow these steps to do the second option.

• Federal Taxes
• Wages & Income 
• Less Common Income select Start (or Update) for Miscellaneous Income.... 

You may not have to pay tax at all if you are essentially hosting an online garage sale.  If you use the site to get rid of household articles you've used in the past, you may qualify for "occasional garage or yard sale" treatment. According to the IRS, if your online auction sales are the Internet equivalent of an occasional garage or yard sale, you generally do not have to report income from those sales.

Reference: https://turbotax.intuit.com/tax-tips/self-employment-taxes/a-tax-filing-factsheet-for-ebay-sellers/L7h6uJr0i which says, in part:

"Assuming that you originally bought the used items for more money than you are selling them for, you don't have to report the income received from the eBay sale. For example, if you sell a bicycle that you paid $500 for two years ago for $350 on eBay, you usually don't have to notify the IRS—but you can't claim a loss on it."

Yes, there is some risk, you'll hear from the IRS.  Keep records of what you sold. 

Q. Is there a way to report it without claiming it as an income?

A. Yes. You have to use a workaround in TurboTax.

Notice that there is no specified place, in TurboTax (TT), for a 1009K.  That's because you are supposed to know what type of  income it is for.

In TurboTax (TT), enter at:
- Federal Taxes tab (Personal in  Home & Business)

 - Wages & Income

Scroll down to:

-Less Common Income

      -Misc Income, 1099-A, 1099-C

       - On the next screen, choose – Other reportable income  

       - On the next screen, click yes

       - On the next screen, you'll get blanks to enter the amount and a description. It will go on line 8 of Schedule 1 as "Other Income".  Suggestion for description: erroneous income on 1099K

When that's done, do it again (a 2nd entry).  This time make it a negative entry; put a minus sign (-) in front of the amount. Suggestion for description: erroneous income on 1099K offset