Let's step back.
First, regardless of any forms, you only have taxable income if you sell items for more than you paid for them, or for more than your cost basis. (Cost basis is usually what you paid for an item, but can be different for items that were gifted or inherited or previously used by a business. Ask if you want more explanation.)
If you are in an ongoing trade or business, you report your business income and expenses on schedule C. Being in an ongoing trade or business means you actively and regularly participate, you operate in a businesslike manner, you seek new business opportunities, and you have a profit motive.
If you are a "Casual seller" then you probably aren't involved in "ongoing trade or business" so any net income is hobby income, and you can't deduct expenses. But remember, you only have income if you sell items for more than your basis.
What you should always be doing, regardless of tax paperwork, is keeping accurate records. You want to keep a list of the items you sell that includes a description, how you acquired the item and when, your cost, the selling date, selling price, and selling expenses.
We don't know how the IRS will want taxpayers to account for the new 1099-K forms on their 2022 tax returns. If you keep proper records, and understand what is taxable and what is not, and whether you are a schedule C self-employed business or a hobby, then you will be prepared for whatever the IRS does.
