You can take a look at top used stores below: 

• KEH
• MPB
• B&H Photo Video Used Store
• Adorama Used Store
• Amazon Warehouse Deals
• eBay

Subbing for updates

Like someone mentioned, those big shops are super reliable for peace of mind. But if you are hunting for the absolute lowest price on an a7III specifically, honestly check out the Fred Miranda buy/sell boards. The Sony used market is so much more active than the other brands lately so deals pop up constantly there. I have found prices are way lower than B&H or KEH because you are buying direct from other nerds instead of a retailer... just gotta verify the seller feedback first. Definitely worth a look if you dont mind doing a bit more legwork.

Basically just check r/photomarket on Reddit. Its where I get most of my gear now. It works because the community is really strict about how things are posted. You can see a sellers history through their flair, which tells you how many trades they have actually completed. It is a decent option if you want to avoid those high retail markups. Just make sure you get a timestamped photo of the Sony a7 III Mirrorless Digital Camera with the sellers username and todays date in the frame to prove they actually have it. Only ever use PayPal Goods and Services tho... if someone asks for Friends and Family, just walk away. It takes a bit more effort to verify everything yourself, but you can save a few hundred bucks easily.

Commenting to find later

KEH.

I usually got some used gears at B&H Photo Video Used Store. Their service is great.