I have gotten most all of my cameras and lenses from MPB , KEH, Adorama, and B&H plus a few more that I can't remember. There are some more places to get good used equipment but the ones I posted are the most popular and trustworthy.
I belong to a couple camera forums, and the above businesses are spoken very highly of!
Check the Ugly Hedgehog forum it is extremely busy and very informative and these members are very good at what they do. The bird and wildlife pics are mostly all pro quality.
I am not one of them by the way.
Some day maybe,
PS they have a pretty good classified section also.
https://www.uglyhedgehog.com/