I would dearly love to see a feedback system hacked in here. If I were designing on the fly, I wouldn't make people reply to service requests via PM. I would make them reply in the thread, but make the replies invisible to everyone except admins, mods, and the thread poster.
Then, when the thread poster chooses the service provider, they would mark the provider, sort of like clicking the install button. Then, if the service provider agrees to provide the service, he would click a similar button to indicate so.
Then, when the job is finished and money has changed hands, feedback would be left. Feedback could only be left by people who have "contracted" with one another, but would be visible to all.
Example feedback would be, member x agreed to do this work, and accepted money but never provided the code.
Or, member x did a great job on my hack and charged me a great price.
Or, member Y asked me to code this hack and I did so, but was never paid for my work.
Or, member Y paid me promptly for my work.
Feedback could be challenged to the admins, and cleared if necessary (in rare cases)
But, that's just a suggestion. I speak for me alone.
Amy