There is going to be much more guidance on this from the IRS come closer to tax time at the end of the year. But the bottom line is you don't pay tax on personal property items that you sold at a loss. You also don't pay tax on collectible items that you sold at a loss. Depending on your CPA they may point you towards seeing knives as a personal property item or collectible item. The rules are a little different on "collectible items" since they are not a cumulative amount, E.G. you don't total up how much you paid and then how much you sold. It's on a transactional basis so you would have to pay taxes on any individual items you made a profit on. But let's be honest, most of us are talking about 10-20 bucks at most and most transactions are for a loss. I spoke at length with my CPA about this and she said bottom line is you don't pay taxes on stuff you took a loss on but there wasn't enough guidance from the IRS yet on how exactly it will be reported, what forms, etc, in light of this new reporting requirement.
She also said to be very clear that you do not have to file as a business; people have been incorrectly reporting that all over different sites. That only matters if you're trying to take "losses" against your tax liability that is above and beyond zeroing out money that was lost on the item. At that point it is no longer a hobby and you have to document various things, things get complicated. But again simply reselling collectible items and personal property do not typically fall under that purview. Nor should it trigger any sort of red flags if everything is properly reported and above board. People properly offset 1099 income all the time due to expenses, it will not be a new thing, there will just be a lot more of it in the 2022 tax year.
Just to be clear, I is a stupid knife guy and I am parroting my CPA's responses from our conversation. I am definitely no tax expert, always talk to a professional!