The current rule is that Ebay/paypal basically doesn't have to report your revenue to the IRS unless you exceed 20k in sales or have 200 or more sales transactions.
The new rule starting in three weeks is that everyone who sells more than $600/yr will be getting the form and having that revenue reported to the feds. Thus you will be required to account for it on your taxes as income. Sure, if you have documentation/receipts, you can subtract your cost from the proceeds and only pay tax on the profits, but that would require you to keep great records on anything you might sell someday or risk being up a creek during an audit.
I've only encountered this once, during my great sell-off of 2014 when I sold my MOC Joe collection, my MIB CHUG collection, and a bunch of comic artwork. IIRC, I sold somewhere between 50-60k worth of stuff. Fortunately, most of the CHUG stuff came from BBTS, and the Joes and art I had acquired mostly through ebay or other online paypal transactions and could prove exactly-to-the-penny what I'd paid. But had I bought it locally or via shows, craigslist, or other "offline" means, I'd have had a difficult time accounting for my costs. Even with good records, I paid several thousand in taxes on the profits, but at least it was only 4-figures and not five.
However, current ebay sellers rarely ever pay any taxes on profits. They know the rules and will have multiple accounts in their wife's or brother's name to continue selling under the limits. The new rules will force them to do so or find other means of selling. The worst part is that regular collectors such as ourselves will be hesitant to sell more than one or two things a year for fear of creating a tax nightmare for ourselves. Thus we'll resort to private sales or simply hang onto things. It'll definitely make finding rare things harder because I expect the number of listings to decrease substantially.