there seems to be a difference of opinion on reporting - one opinion
https://ttlc.intuit.com/community/tax-credits-deductions/discussion/1099-k-for-personal-items-sold/0...
another
https://ttlc.intuit.com/community/taxes/discussion/1099-k-amp-personal-items/00/78901 (the link in the thread does not work)
here's another that suggests reporting on form 8949
https://www.lawofficesofjasonknott.com/where-to-report-form-1099-k-on-your-tax-return/
all this shows is no one is absolutely sure how to report a 1099-K for the sale of personal items. the article by Knott does have some logic in that the IRS has no way of knowing that the proceeds were from the sale of personal items sold at a loss. I would use schedule 1 misc income and not 8949. not reporting it could result in an IRS notice adjusting your return. you would then have to dispute the change. addressing the issue upfront should avoid the notice.
others may hold to the opinion not to report it at all.
if you report on your return, the cost would be equal to the sales price since no loss is allowed on the sale of personal items
