Yep, http://www.adorama.com and http://www.bhphotovideo.com are both highly recommended places to buy used gear. They carefully inspect all the used gear before you buy it, and they have excellent support and return policies if something does go wrong. I’d much sooner buy from them than a random seller on eBay or Amazon where you never know what you’re getting.
If you’re specifically looking for Canon gear, definitely check out “Canon Price Watch:” https://www.cpricewatch.com/ They keep track of all the different sellers and tell you where you can get the best deal on any particular item you’re looking for.
Also make sure you don’t ignore the official Canon site! They sometimes have some very good refurbished deals. You can find their refurbished section here: https://shop.usa.canon.com/shop/en/catalog/cameras/refurbished-eos-interchangeable-lens-cameras?cm_sp=Refurbished-Products-_-Categories-_-EOS [BEWARE: the Canon website’s cover image for the Labor Day Sale is not “family friendly,” it is VERY indecent!!]
Here’s a direct link to a FF camera within your budget that shouldn’t have any inappropriate images on the page: https://shop.usa.canon.com/shop/en/catalog/eos-6d-mark-ii-body-refurbished
Do you currently have any full-frame compatible lenses to use with your new camera? If not, do you have any Canon lenses at all?