Needed to make extra money this year and sold a bunch of stuff on eBay almost all at a loss but none of the items had receipts - many old items. How do we verify they were at a loss if audited?
With difficulty.
You should have been making (or make now while it is fresher in your memory) a list of all the items you sold, including an accurate description, the date you acquired the item, the approximate price, the date sold, and the price. Also keep copies of the eBay listings showing you advertised the items as used, and not new.
The IRS does not have to allow any tax break that you can't prove with adequate reliable records. Reliable records can include your own handwritten or typed list, records or inventory, as long as you wrote that list close enough in time to the event that your memory would be considered reliable. There's no guarantee an auditor would accept your list, but in combination with your listings, and assuming everything seems reasonable (you didn't sell 10 of the same dress, for example), then an auditor can choose to be reasonable.