My wife makes money on Ebay. She likes to shop, and this gives her an excuse and she actually turns a profit.
She has a knack for finding things that people want. For instance, she picked up a whole case of foaming baby shampoo at Kays for like fifty cents a bottle and sold them at $20 a bottle. I always wondered if there was some illicit use for the shampoo, I mean, $20 a bottle!
She goes to sales and picks up discontinued items for $4 and sells them for $100. She likes to go to second hand stores, auctions, and yard sales and can spot valuable items,
Anyway, it's just a matter of buy low, sell high. But you really can't sell the same item for long because, as someone has already mentioned, when people see something sells, they jump on the bandwagon.