1. Not everywhere: If there's too many people selling the same or a very similar product on ebay you won't make a good profit margin. You also risk a price war where sellers will bid down the price in order to get more sales, reducing their margins to compete, and ultimately loosing money.

2. Small and light: This works in two ways. It will cost you less to get the product from your wholesaler to you, increasing your margin. It will also cost your buyer less to get the product shipped to them making them more likely to purchase it.

3. Perceived value: If you can convince people that your product is worth more then it actually is your margins can be very high.

4. Expensive: Expensive items tend to have a higher profit margin available. However buying a significant quantity can be difficult, so cheap items with perceived value are also a good option. Remember selling 10 items for $5 profit is harder then selling 1 item for $50 profit. Selling less items will also keep shipping and ebay fees low.

In summery, the best item would be small, light, rare, and expensive, but where its perceived value is much greater then its actual value. Good Luck.