There are two primary websites amateur astronomers use A LOT:
CloudyNights.com and AstroMart.com
CloudyNights.com is free. AstroMart.com used to be free, but now has a variety of membership options starting at $15/year.
Both offer similar content, include Classified Ads for buying and selling used equipment. Both have a ratings system. Sellers and buyers there want to maintain their 5-Star rating, and a long history of excellent reviews indicates that a seller is likely to be trustworthy. One could argue that AstroMart buying is "safer" because the fee weeds out lazy scammers.
I've heard arguments that because of the fee the AstroMart forum's activity has decreased over its free-access days.
If I have a question about equipment, there is generally a review, an article or a discussion on CloudyNights. It's my GoTo source for pretty much everything.
There are a lot of other sites with forums, but those are the two biggies.