You are supposed to notify SS of any business earnings. So, if you regularly sell on Ebay, or are buying items for resale, than you need to let them know.

If you have a garage sale or sell some used personal items on Ebay a few times a year, that probably doesn't fall into the scope as if it's a business.

If it takes you 20+ hours to list, pack, and mail your items--that's probably a grey area that might get you into trouble if you don't notify them.