Firstly choose an area of interest - in other words if you like mobile phones maybe you could sell a case or other type of accessory. Selling something you're interested in makes the process a whole lot easier and more fun. If you have some background knowledge as to something's worth, then you can use that by combining a google search with a search on ebay to find out; a) what the item is worth on ebay (the public's perception of the value) and b) the actual lowest cost you can buy your stock for. In lower margin items you need to pay careful attention to your shipping costs, listing fees, sale fees because this can outstrip your profits quite quickly (especially if you need to relist). Look to Google as a solution, utilise key word searches until you find a website that looks reliable and meets your businesses demands, and only then consider the business as a possible supplier. Remember, if it looks too good to be true then it probably is. Unfortunately, a lot of people get caught up buying fake goods because the price is so attractive and end up discouraged from future selling endeavours.
What have I sold? Well. over the years I've sold hundreds of different things. I started off importing region 1 (USA) DVD's for sale in the Australian market. This was very lucrative in its early days, until eventually more people caught on and it became like anything. There's still money to be made in this market here in Australia. A lot of people don't trust overseas DVD sellers and are happy to wait for you to order the stock in, then pass it onto them. This is called selling on a 'pre-sale' basis. You have around 20 days to get the product to the ebay buyer before they can legitimately start complaining (of course you must advise them if there will be a wait on the stock in your ad). That means you can use the customers money to purchase the stock, then keep the profits, particularly good if you dont have a lot of spare cash to outlay. More recently I've sold computer parts, gadgets and now mobile phone accessories. I will say that if you live in Australia and don't have access to wholesale prices, don't bother with anything technology as it's simply too difficult. If anything, keep an eye out for specials on mobile phones (you can do the pre-sale thing again, just make sure you can get the stock quickly), as they sell OK. As time goes on this is becoming harder to do.
Because I'm advising you to sell with limited cash outlay, you can generate better returns than if you were to purchase a stock on the stockmarket and be lucky to have that increase by 15% over a year. Stick to profit margins of at least 20% for higher priced items ($150-300) and you should be able to generate a good income for yourself. Currently I'm dealing in DVD's and am looking at returns of 30-40% per item sold after all costs. Be creative, look high and low for the best price, and try and find new untapped markets.