If audited, the IRS does not have to award any cost basis you can't prove.
If you have an inventory of the items (type, date, condition), you might be able to find some 30 year old coin and stamp valuation guides and check the values. A dealer might save old guides, or maybe a library would have them. That would be better than nothing and would give you something to stand on in case of audit.
But if you don't have a detailed enough inventory, you are probably stuck reporting the entire proceeds as a gain.
