There are two types of buyers on eBay: those that sell stuff as they would in a garage sale and those that constantly buy and sell merchandise for profit. If you're from the first category you don't have to worry too much about what products to sell: you only have one or a couple of items any way.
If you are selling products for profit, however, there are some strategies that can help you generate increasing profits. There are also two categories of "professional" eBay sellers: the ones who buy large quantities of products and resell them and the ones who deal with original, hard to find, niche products. Let's have a look at the first category:
Usually, when you buy a large quantity of identical or similar items, you get a discount. This is true with most products and it stands at the basis of numerous eBay business strategies. You can then take the products and list them for a higher price than what you paid, but also lower than the prices available in stores. Finding the right balance is essential, as you want to generate enough profit without listing a discouraging high price. You choice of products to sell is very wide here, and specializing on a certain type of merchandise is usually dependent on your personal choices.