I sell on
ebay, (but can't tell you who I am until I get 50 posts).
The general rule of thumb people use is to only bid one grade lower than advertised. If you stick with this, you might get some good deals every once in a while.
Also, when I list coins I am not interested in keeping, I generally list them at half their posted value in
Coin World's coin values magazine (a montly publication that updates values).
Usually these are coins I buy in larger collections that I have no interest in. I don't need to make much more than my money back that I spent to buy the collection plus I get to keep other good coins from the collection, so I generally let these go at a discounted rate, just to move them and get my $ back quicker, so the answer to your question is yes, you can get some good deals as long as the guy or gal has good feedback.