In the forums that allow the buying/selling prefixes, there should be a way to display only buying or selling threads. This should also be a function when searching for a particular string inside of a forum. For example, if I am looking to buy a link in the gardening niche - I could go to the Link Sales forum and search Gardening [display selling only]. This would display only the threads of users selling links in the gardening niche. It would make the experience a lot more user friendly.
You can... when looking at the list of the threads, click any of the prefixes you want to filter by. You can also do it by going to the bottom and using the "Thread Display Options".