DiscGolfReview.com forums...been trading there for almost 3 years, never had a bad experience.
Never tried discaroo myself, but many of the guys who trade regularly on DiscGolfReview has been burned by people on discaroo, so I don't ever plan on using that site.
The PDGA forums has a buy/sell/trade section there too...I'm not a member anymore, so I don't currently use it, but when I did it always worked out well. If you've got rare/collectible stuff, the PDGA forums or Ebay are probably your best bet.