I would say about 50% of my wardrobe is off of ebay. I love it when people say 'I love that skirt where is it from' and I can reply "99p off of ebay". You can get some great items. My favourite is a faux fur crop puff sleeve jacket in animal print that I picked up for £10. My tip is - never spend more than you would be prepared to lose, just in case you don't like it or it doesn't fit. (though you can always sell it if you don't want it). To start with, why not try searching for items in your size from a shop you know. E.g. I often type in 'Topshop size 6' and see what comes up. You can usually get some current season things, and some older, vintage style clothes. It's great for unusual finds. As long as you don't mind wearing second hand clothes - it's kind of like charity shop shopping without the musty smell and childhood associations. My friend even got her Vivienne Westwood wedding dress off of ebay. One word of warning though - it can become addictive! Not handing over cash or your credit card each time, it can become easy to lose track of how much you're spending, especially with the wonder that is Paypal (where would I be without paypal), so make sure you keep checking your credit card statements.
My latest ebay bargain is this fab Miss Selfridge dress which I bagged for the bargain price of £4.76 including postage!