Here is how I found WebHostingTalk.com, when I first wanted to sell a hosting name I had:
I went to google and searched for Hosting +forum..many times you have to search for hosting +talk.
Similarly let's say you have a XYZ industry domain so best thing to do is go to google and search for XYZ +forum or XYZ +talk or XYZ +discussion
I have found that selling domains is easier in a industry related forum then it is to put a selling page etc. Also, you don't exactly get higher prices for them but one can make decent profit
Good luck!