Do I need to amend?

Yes, since you already filed, you must amend. You must report your income. 

Since you were not in the business of selling, you can report it following the steps provided by @PattiF.

This can be reported as the sale of items not associated with a business so this won't be considered as self-employment income

In order to do that you can either report it as investment income or other miscellaneous income. Make sure to include expenses of the sales and the original cost of the items.

For reporting Form 1009-K for personal items sold not associated with a trade or business, you can report this as Miscellaneous income.

• From the left menu, go to Federal and select the first tab, Wages & Income
• Add more income by scrolling down to the last option, Less Common Income, and Show more
• Scroll down to the last option, Miscellaneous Income, 1099-A, 1099-C and Start
• Choose the last option, Other reportable income and Start and Yes
• Enter the applicable description and amount and Continue
  • First, enter Form 1099-K as received. It is essential that the full amount be entered.
  • Next, enter an adjustment as a negative number to reflect the cost of these items and the sales expenses