I guess I assumed it would be handled through ebay's payment system. If not, I could see how that would be a pain. Still, that's only 50 checks a year. Doesn't seem that difficult.
It's not the 50 checks a year, it's knowing 50 state laws (which are not all the same with regard to what is and is not taxed, and can and do change), tracking, computing, and being ready to prove to 50 jurisdictions that you've complied with their laws. If so much as one of them decides to challenge you on it, you can probably kiss your profits for the year goodbye. Merely writing the checks is trivial.
What about everyone that sells online and doesn't do it through eBay?
You go out of business because your margin was razor thin. Obviously.
[deleted: don't feed trolls]
If that's the only argument then I hope it passes. Why should some minuscule number of small businesses hold everything up?