I've never asked for or been asked for an invoice. I can understand a business buying from a business for bookkeeping purposes, but what are you expecting an invoice to do for you in a private sale? Protect you from fraud somehow?
Beginning this year, third-party payment processors will be required to report a user's business transactions to the IRS if they exceed $600 for the year. The payment apps were previously required to send users Form 1099-K if their gross income exceeded $20,000 or they had 200 separate transactions within a calendar year.
Democrats made the change in March 2021, when they passed the American Rescue Plan without any Republican votes. Now, a single transaction over $600 will trigger the form. The change is intended to crack down on Americans evading taxes by not reporting the full extent of their gross income. However, critics say that it amounts to government overreach at its worst and that it could ultimately hurt small businesses.