Not sure if you know this by now, but if you are contacted outside of Undev but still want to use it to conduct the sale then use their 'Add Lead' option (in Sales Activity area) and the commission is 5% instead.
Problem with the selling domain names is this: You are "most of the time" dealing with a clueless average Joe who has no knowledge of escrow, transfer, nominet etc. I used efty.com but quit because when you agree on a sale you are alone with the potential buyer and need to explain them all before-mentioned stuff.
The basic rule should be if you care about the magnitude of the sale price. If I am dealing with a GBP500 guy I don't care about him. He can wait he can cancel or get lost. However if I am dealing with a GBP5000 possible sale I use my words wisely and ready to pay 9 percent commission happily if I am guarded by undeveloped.com.