Hobby Sales generating 1099

I am a hobbyist and am retired. I have reached the age where I need to start liquidating my vast array of parts purchased at flea markets and garage sales.
Last year I sold less than 200 lots totally just over $2,000.
The problem for this year I have no memory or documentation of what I paid for these parts, sometimes over 20+ years ago.
I dread the thought of paying taxes on the gross amount but have no idea on the cost of goods.
I know that some items will sell for less than I paid and some more.
I will know my EBay and PayPal fees which can offset the gross amounts.

What do I do for cost of goods?
Will I need to pay any Self Employment tax?
One suggestion I saw posted was to just list an amount as a negative equal to the gross sales.

Thanks