It depends on your recording keeping.
There are two separate issues here.
First, should this be considered a hobby or a business. The IRS has some guidelines.
https://www.irs.gov/newsroom/heres-how-to-tell-the-difference-between-a-hobby-and-a-business-for-tax...
Second, how good are your records.
To put it bluntly, if you don't have records, the IRS can tax it all. There was a court case of an IRS agent (!) who did what you did, did not have records, and the IRS taxed it all.
You should have, at a minimum, a list or spreadsheet showing the item name or description, date purchased, price when new, date sold, selling price, and other costs (listing fees, PayPal fees, shipping fees).
If this is a hobby, you only need to report items you sold for more than your cost, that results in a taxable capital gain (using the interview for selling property and other investments). Don't report items you sold for a loss because you can't deduct it. If you get a 1099-K, you need to report it, and I think there is a button to click for "I was selling personal items for less than cost" (because this is a common situation). The income won't be taxable, but if you are audited, you need to show those records, and they need to be made close in time to the sales, and not the night before the audit. (Make your best guess where you don't have proof.). The IRS is not going to expect to see a receipt for a 10 year old piece of costume jewelry, but they will want to see a description that justifies the price you claim. (Don't say "handbag", say "Michael Kors handbag from Macys in 2009, approximately $200 new".)
If you wanted to treat it as a business, you would list the items as inventory, and you can deduct your selling expenses as a business expense. However, since one of the factors of running a business is profit motive, and selling only used items for less than their cost will never result in a profit, treating this as business will probably not survive audit, unless you are also doing something like browsing garage sales and thrift stores for hidden gems that you can mark up and sell. That would result in a profit and might be allowed as a business.
