No records equals zero basis.

When you tell them that they usually come up with something.

I did submit family photos in an IRS audit on sale of home. They moved to a new tract of homes and the field mice ate most of their receipts. We have original purchase documents and photos of the backyard without the addition and with the addition. The old kitchen at Thanksgiving and the new kitchen years later. IRS did accept reasonable estimations of work done over the 30+ years the client lived there.
I had another audit with a plastic bag full of moldy receipts. I kept them in a banker box in preparation for the audit. Yellowjackets nested inside the box and added to the grossness of the documents. IRS settled for a reasonable estimation (Schedule C, cost of goods sold paid in cash at a vegetable stand).
No records does not mean NO records.

Sure, George, but YOU can't make up the numbers.