1099K from eBay - Sales of Personal Items/Collectibles with No Purchase Records

I sold quite a few personal items on eBay last year (mostly model railroad supplies), and I've received a 1099K (gross sales were roughly $10K). 

 

The problem: most of these items were acquired years ago at shows, where I paid in cash or traded other items. I have no record of what I paid for each one. Without records of what I paid, how can I file my 1099K without overpaying taxes on the full $10K? Of course, eBay fees, shipping costs, etc. can be easily deducted, but I'm not sure how to account for the value of the items themselves. Any help would be appreciated. Thank you!