I am a 72 yr old retiree and have been selling items out of my closet that I no longer need since I am not working. I realize I will need to file a Schedule C for 2022 but I do not have receipts for these items. Will I need to pay taxes on revenue if I cannot prove their original cost?
Selling items from your closet should not be reported on a Schedule C unless you are actually altering the items and you are creating a business of selling your clothes.
This is a sale of personal property should be reported on Form 8949 that the goes to Schedule D (this is taxed slightly differently, avoids added self-employment tax and gains can qualify for long term gain preferred tax).
For you cost ideally you would have some type of receipt but as long as it would be reasonable and you can document it then you should be okay.
For example, I have a ton of purses. I know that in general I pay around $30 to $40 for any purse. I've owned all of these more than 1 year. If I were to sell one I would use the purchase date of "various" and when asked I would say it was "long term" and I'd enter $35 for the cost. In my records (I keep notes with my return) I would make myself a note as to why I did this. The receipt is only required if you are audited by the IRS.