I think as long as you do your research and set a maximum you can get bargains, but you also get alot of failed bids.
You just have to keep at it. I have bought several loco's on eBay as low 50% of the trade prices, but you get the odd idiot who trys to sell a Hornby as a Wrenn.
What suprises me is where do they get there stock from? they can't all be dealers off loading can they? rails of sheffield for instance and even there you can get an odd bargain. missed 3x Commonwealth of Austraila boxed set sold for £140 amazing value.
Mike