Its not required. eBay wants social security numbers in order to be able to report sales to the IRS. The stuff you buy new will be depreciated so that selling creates a taxable gain. Or even better, if you can't document what was originally paid then the whole sale is taxable income. Then if you buy and sell more than a few times guess what? Now you're a business. You'll need to buy a license, keep records, pay taxes. Think I'm kidding? Its exactly what they've been doing with cars for years.
Done selling on eBay, they are demanding my social security number to sell
They are forcing everyone into what they call "Managed payments" that requires you to give them bank account information and your social security number. Looks like I'll be selling here from now on. I know I gave it to PayPal years ago so they have it, but I'm done giving it out. I called and the rep told me they are legally required to have it in order for me to sell. That is a load of crap.