What to Sell.
First off, you need to know what it is you're going to sell: what's your speciality? You'll do far better on eBay if you become a great source for certain kind of products, as people who are interested in those products will come back to you again and again. You won't get any loyalty or real reputation if you just sell rubbish at random.
When you think about what to sell, there are a few things to consider. The most important of these is to always sell what you know. If you try to sell something that you just don't know anything about then you'll never write a good description and sell it for a good price.
You might think you're not especially interested in anything, but if you think about what kind of things you usually buy and which websites you go to most often, I'm sure you'll discover some kind of interest. If all else fails mention it to your friends and family: they'll almost certainly say “Oh, well why don't you sell…”, and you'll slap your forehead.
Out of the things you know enough about, you should then consider which things you could actually get for a good enough price to resell, and how suitable they would be for posting. If you can think of something of that you're knowledgeable about and it's small and light enough for postage to be relatively cheap, then that's great!
Don't worry if you think the thing you're selling is too obscure – it isn't. There's a market for almost everything on eBay, even things that wouldn't sell once in a year if you stocked them in a shop. You'll probably do even better if you fill a niche than if you sell something common.