Most people set up BuySellAds so they don't have to deal with people directly. They also do it to get in front of eyeballs they otherwise wouldn't, through the marketplace. Some of them might be excited to not have to give a cut to another company. You may not get any discount doing that if they get dollar signs in their eyes.
People definitely have success or the marketplace wouldn't exist and be sustained. But like anything you have to set up your tracking so you can test everything and get rid of underperforming media buys. You'll have to burn some cash to get the data.