Here's some ideas to help you determine what to sell:
1. Sell what you know first. If you're into fashion, sell that; if you're into golf, sell golf equipment, and so on.
2. Be observant. Watch for opportunities to obtain items (in quantity if possible) at a low price for resale. Often, they're right under your nose, such as merchandise your local retailer would liquidate. Make an offer to clear out the re-tailer's old or damaged stock.
3. Don't start out with just one product or product line. An effective way to build a customer base on eBay is to offer at least two separate types of items, between which you can cross promote and drive customers from one to the other.
4. Spend time on ebay browsing outside your normal categories. Find out what's hot at www.ebay.com/sellercentral.
5. Become a trading assistant and sell other people's items for a commission. As a Trading Assistant, you won't have the expense of building inventory, and you can build your business on eBay very quickly without having to spend a lot of capital.
6. Take some calculated risks. Being too careful can cost you. Do a little research on eBay, and trust your gut when you find potentially salable items.
7. If you're not familiar with the product you're trying to sell, educate yourself before you write the description. If you describe the item incorrectly, buyers may be leery of bidding. Even worse, buyers could decide not to bid on any of your future items if they're under the impression that you don't know what you're selling. The more correct information you have and use, the more credible you'll become in the buyers' eyes.