Some are genuine dealers in things like antiques or whatever - they just use ebay as a platform to sell their goods. Other people scout around car boot sales and even ebay itself for bargains and then re-sell them. If you know your market, there is money to be made without stealing.
I knew a guy who was offered a load of cheap trainers as they were all factory seconds (about 200 pairs) - he sold them all for a £5 profit which made him a clear £1,000 for very little effort. Took him about a month to shift them. I'm sure there are lots of other ways too...