1099K from Facebook and eBay sales

Hello!  I am a collector of action figures and have amassed quite a few over the years.  I routinely had bought and sold (usually at a loss when it was all said and done after fees and shipping) compared to what I usually paid.  I know that since I collect and sell high end (expensive) figures that I will hit the thresholds.  What do I need to have prepared to show that I took a loss on these?  I intentionally don’t sell over what I paid for things so that I don’t take a profit as the intent of me selling them is to buy different ones not run a business or make a profit.