1099-K from PayPal (tax form)

If you can remember what the things cost you then you can deduct the cost since you already said you lost money.  If you lost money, you don't owe taxes on the sales.  You just need to deduct what you originally paid on them (since you most likely did not take depreciation against your taxes in the past).  It's a pain but if you sold more than $20,000 AND over hit 200 items sold, then you have to deal with the 1099

If you sold over 200 items that probably was not just hobby stuff. Talk to PayPal unless you have already deduced what you did.

The 1099 only reports proceeds.  Proceeds minus basis  = profit (loss) on the item.  The problem with the sale of personal property is that not to many people keep evidence of their basis in everything they own.  Making this worse, you have to declare the "profit" in the item sold for more that you paid for it, but you cannot declare a loss for items that sell for less.  There is no offsetting of losses in the sale of personal property.  I'd like to know what a tax professional would tell you to do.  Estimate the basis?  

I sold 3 used audio items for $630 total in 2020 and Paypal just issued me a 1099-K.  

I'm guessing its because the buyers asked me to send them a "invoice" (i.e. not Friends and Family payment option) so because of that Paypal treated the transactions as business sales. None of the other dozen or so friends and family transfers from 2020 appear on the 1099-K.