Where can I enter seller fees and shipping labels as an expense?

You can enter those expenses on the Miscellaneous section. See images below for reference 

• After you have entered all your income, and main information related to your business, you will select Add Expenses for this Work

• You will see a few expenses categories already populated for you based on the type of job you have. Any expense that doesn't fit in a populated category, you can enter it under miscellaneous.

If you are reporting the income as hobby income, you can't deduct expenses.

If you are reporting your eBay activity as a business, you need schedule C for self-employment.   You report all your gross income (even if not on a 1099) and you can list your legitimate ordinary and necessary business expenses.  See the other expert for where to enter your expenses.