No bones about it: eBay can be a great place to find nice clothing for cut rates. Suits from brands like Hickey Freeman, Brooks Brothers, and J. Press filter through there regularly. I like the "Saved Searches" notification feature. Set up a category you want to monitor, and eBay emails you when something new appears in the category.
However, you have to be savvy. You have to know exactly what you're looking for. You also need to be smarter than the sellers. A lot of these people are not clothing experts. I recently bid on a "suit" only to realize when it arrived that the seller thought that "suit" meant "matching jacket and vest." Sellers routinely abuse "vintange", "mod", "indie", and the sizing parameters, so while you can narrow your search to, say, 40Rx34, there will almost certainly be at least one 40L in your search results.
The flip side is that volume sellers rely on 100% positive feedback, so they have a strong incentive to keep their customers satisfied.