No suggestions offhand for buying and selling, but I have a couple for repairs.
I don't know where you are located, but Strauss Photo-Technical in Washington, DC is an absolutely topnotch place that I've used several times over the past 35 years. I'm sure there are other good ones, but these guys have been able to fix problems and locate oddball parts that nobody else has been able to find.
http://www.iglobal.com/SPTS/
If you need service for professional grade camcorders, I recommend Robb's Repair
http://www.robbsrepair.com/ near Seattle.