After you file

Yes, eBay fees are deductible.  You could consider calling them "selling expenses" rather than commissions, but if you had a taxable sale then you can deduct them to offset the income tax being generated.

If you have a business, include the expenses in Schedule C.

If you sold personal items, like concert tickets, after you first report the 1099-K, secondly, you will need to report the details of the sale. In the investment and savings interview, enter the details of the eBay sales.  Continue until you reach the "Let us know if any of these situations apply to this sale" and then enter the fees.  Note, if these were personal items sold at a loss, you should indicate this in the original Income from 1099-K interview because that is not taxable.