I love Rokslide classifieds, but if your looking for name brand used gear, buying on a site full of people who tend to really know name brand gear is not going to land you the best deal. But, on here the seller is probably going to be able to awnser the more technical questions
Look on local Facebook or Craigslist adds. People also seem to be more responsive to lower, but not lowball, offers. They just may not understand super technical questions.
*disclaimer* I realize I'm speaking in generalizations here...
As for used vs new, I will almost always look for used first. If you have time to search and wait you can land some great deals.