This year and going forth the US law has changed on what gets reported by companies like eBay and Reverb who handle the payment for a sale. This also applies to services like Venmo and PayPal.
I won’t go into detail in this post, because frankly i don’t know that much detail. I will say that largely you only owe taxes on profit, the same as in the past. It’s the reporting, the 1099, that has changed.
I am starting this thread to give this topic a home, and get it on-topic.
Here’s an article from CNBC that seems a pretty easily read yet a somewhat complete summation.