I would say that eBay makes up well over half of my clothing purchases. I kind of begrudge paying full price for something when I know I can get it at a snip by sniping an auction. I swear by it. It helps if you are quite specific in terms of what you're looking for. Eg a certain brand of jeans or a model of a piece of knitwear. At very least, the kind of item that you're looking for rather than jus, say, 'jacket' or 'jeans'. As Ben says, you used to be able pick up All Saints stuff for peanuts if that's your bag.
I haven't got any hot links for stuff at the moment (apart from the stuff that I'm watching- sorry) but I'll give you my top hot purchases in no particular order.
Edwin ED62 Dry Kozan Hidetora. Brand new. RRP £95. I paid £25.
Studio d'Artisan Selvedge Raw denim Jacket. Brand new. RRP £300. I paid... £10
Jaegar herringbone tweed blazer. Vintage-still in excellent condition. Jaegar blazer RRP £200 ish. I paid £22.
Hudson Brogue boots Worn once for a shoot. rrp £110. I paid £27.
Nudie Dry heavy Average Joe. brand new. RRP £100. I paid £24.
Vintage Shearling jacket. RRP £200+? I paid £50. Doesn't look like it has been worn.
Vintage Dior prince of wales blazer. RRP £200+? I paid £18.
Countless other items.
In short. If you don't know exactly what you're looking for then you will need to be patient!
Make sure you ask for as much detail as possible, especially when looking at tailored items.