As a CPA part of my job with the large companies I worked for was to be the liaison to the IRS when the IRS audited us. In fact, most large companies such as mine had established permanent office space in our building for the IRS as each tax year is normally subject to IRS audit.
It has been my experience that the IRS is a very reasonable group of people. In fact most I met are really great people, audiophiles, hunters, our neighbors In more years than I will mention I never met one out to screw anyone. If you can verify that the numbers you have put on your tax return are correct, they will accept it. In fact, an IRS agent that can be shown to be acting incorrectly will most often be disciplined and likely terminated.
Bottom line, if you can show in the event of an audit that the amount of taxable income you have on your tax return is correct, you will prevail.
Now I am only talking about us hobbyist selling our personal audit gear, the problem is the 1099K is only showing the IRS how much money you sold your items for. But the IRS only wants you to pay tax on any profit, the excess of sales price less selling expenses over your cost. The BIG PROBLEM I would have in this case is finding the invoices showing what I paid for the equipment to prove I did not make a profit on it. I don’t think I could for a lot if not most of my older gear.