Q. How do you calc cost basis for an item purchased 30 years ago?
A. From your own records or memory.
For personal items sold, you usually do not need to report the sales. For a full discussion, on that topic, see: https://ttlc.intuit.com/community/taxes/discussion/i-sold-some-personal-items-on-ebay-and-received-a-1099-k-i-am-in-mass-where-the-limit-is-600-i-made/00/1831439
Whether "Hobby income" or the sale of personal items, you must report any profit you made, but can not claim a loss. You may not offset the loss on the sale of personal items against any personal items sold at a profit. But, you can offset "hobby" losses against hobby gains and report on the net gain