I think the tax reporting thing is what's driving a lot of people to really want F&F. Goods and Services will trigger a 1099-K. It's been delayed a couple of tax years. Amounts that trigger that appear to be changing (going up). It seems fluid and a new administration is headed into office soon.
Selling something at a loss (used sporting goods) wouldn't normally be a taxable event. And you couldn't claim it as a loss either (reduce taxes).
TBD how much backup (original purchase receipts???) you need to prove you didn't sell at a profit. Or, if you could just attest that you're selling used stuff at a loss and say it's related to a non profitable hobby or 100% personal? Who knows, what will actually happen… In practice vs. big scary IRS rules.
If you are selling at a profit, and it is a profitable side business, you may find ways to write off a lot of expenses related to your side business. Math, taxes, keeping paperwork isn't a whole lot of fun. I get paid to do it and I don't like it.
Selling at a loss (likely) + paying shipping + PayPal fees. There probably an incentive to get out of the PP fees and that makes sense. I think for a lot of people it's the tax reporting thing. As a buyer, the "protection"is very nice. But as others have said, the buyer may have none because it's likely pew pew related.