I never thought making money would be this easy. The best piece of advice I could give to those starting out is to study the ebay site and see what is attracting the most amount of bids. I did, and noticed a demand for vintage style clothes. People love the idea of buying one off's, but don't want the hassle of searching through op shops on Saturday mornings. So I did it for them. Offering buyers a unique top or dress that I had located and paid a reasonably small price for, but because most times the item is a one off, buyers are prepared to bid with their heart and not their wallet!
If clothing isn't your thing, then find out what is. You have to enjoy and know your product to make it work for you. I sell a handful of items a month, which in turn makes me a few hundred dollars richer every month. I know that if I wanted to take it seriously I could make a real business of it...and who knows maybe one day I will. But for those starting out...here is my advice..
1) Make the ebay website your new best friend- it is all the research you will need to do before you begin selling
2) Learn your product- buyers are savvy and want detailed answers to their questions
3) On that note, be accessible, be able to answer questions and post items quickly. It will mean your buyers will keep coming back if they are treated well
4) Have fun with it- don't limit yourself to one product
5) Open a seperate account where you can store your earnings and before you know it, you'll have enough money to spoil yourself for all of your hard work.
Good luck and happy selling!