It isn’t (supposedly) just single transactions - but the total amount for the year.
You sold your old gym sock collection for $1/pair... and over the course of the year - all 30 pair were sold - that’s $30 (plus anything you included in the sale for shipping)... so say $60 with shipping. You also sold that old generator for $400 (that you bought for 1200 a few years ago). And you sold the old, nearly bald tires off your truck when you bought new ones - and sold them for $200. Ooops.... you now are $660 - 10% over the reporting threshold. Venmo (or Paypal, or whatever payment processor you used) auto-generates a report to the IRS for that $660 in “income” you received for the taxable year.
Now- you MIGHT skirt this if you use multiple services to handle payment - say the generator via a completely different entity than the tires or the sock collection... for now....
Cash is still the way to go and in our experience most people are cooperative when I say cash only on a transaction