1) The Online Garage SaleYou know how a garage sale goes. You dig through your old clothes, toys, electronics, etc. and you drag them out to your garage and driveway to sell to your neighborhood. Maybe you'd like to get $30 for that old-but-still-good wooden table, but someone offers $10, and you go with it. This is one business idea for eBay, and it may be the most common for casual eBay users.
The benefits of this model is that it usually doesn't cost you anything to run. You're selling things you already own, and anything you make is helpful as long as you can cover the time and cost of dealing with shipping. Furthermore, you can gather unwanted items from friends and family and profit from those as well.
The downside is that you might very likely run out of products to sell, and you don't have the ability to replenish this supply once you've sold all of your own unwanted goods (and those of friends and family). Profit potential with this model is very limited, and this would likely only make a good side-business (perhaps one that you spend a few hours on each weekend).
2) The ResellerAnother way that lots of people make money on eBay is by purchasing products for one price and selling them at a higher price. To do this successfully, you need be able to find a reason why you'll be able to sell a product for more money than what you paid.
One way to do this is to buy an entire lot of products (i.e. a large quantity) and then sell them in smaller pieces. Once you sell all of the smaller quantities, you should end up with more money than you paid for the entire lot. For example, you might buy a package of 50 DVDs for $100 ($2 per DVD) and resell them for $5 per DVD, individually. In this example, you would net a profit of $150. Tip: Charge a little bit more for shipping than what you expect to pay - this will give you a little bit extra profit margin.
Another way to make money reselling is by purchasing a product, and fixing it up to resell. You'll find some people selling a table that says "legs are a little bit loose" which automatically screams "low quality" to many buyers and will allow you to buy it at a deep discount. It may be as simple as tightening some screws, cleaning the table, and then being able to resell it in "excellent condition."
Similar to the above idea, you can also find auctions with poor ads and buy them to resell with very nice ads. A lot of people fail to use pictures for their products, which really keeps the price down. If you buy the product and take some really nice pictures of it, you can post these with your ad and be able to sell the product for more money.
Finally, you can purchase a product from somewhere else to resell on eBay. A great place to do this is on Craigslist, where people are just trying to get rid of their goods at any price. Use your negotiation skills to purchase products for extremely low prices (from people who simply want to get rid of them), and then resell them on eBay for a lot more money.
The benefits of the reseller idea is that you don't need to rely on products you own. You can literally find ANYTHING to resell and it doesn't take much time at all.
The downside is that you run the risk of not being able to sell a product for more than you paid. Even worse, if you can't sell a product at all, you may get stuck with it for awhile and need to have sufficient space to store these products.