This isn't really new, although the threshold now is much lower than it used to be. You were always supposed to report sales like this on your taxes. If you've never done it before, you might have been breaking the law. Oops.
The eBay change makes it easier to do this, honestly. I used to keep track of all this on a spreadsheet, and sometimes I would forget stuff. It will be nice to have someone else keeping track of it.
The good news is that you only owe taxes on
profit, not sales (even though all sales over $600 have to be reported). If you bought a marker a few years ago for $600 and then sold it for $500 this year, you won't owe taxes on that. If, however, you've been making your own parts to sell, or you're buying wholesale and then reselling to a store, that's different. You're making profit on that, which is income, and so it's taxable. Because it's income.
So, all you have to do is keep track of how much you paid for stuff. Once you plug in those numbers while doing your taxes, chances are you'll owe no additional taxes, or very little. I've made a few thousand dollars making SPAR valves over the last few years, for instance, and I paid less than $100 in taxes for all that once I deducted all my costs for raw materials and whatnot. Considering all the money I've made, that seems more than fair to me.
(By the way, if you're scared of giving eBay your social security number because someone might hack into their files and steal you information, you should really
set up a credit freeze with the major credit reporting agencies. This is a little bit of a hassle. It takes some time to set it all up since you have to do it with each of the three big credit bureaus. But it is free, and once it's all set up you really have complete peace of mind. I did it a few years ago when there was that huge Experian hack, and I've never regretted it.)