I've never bought a used Mac, was thinking about a Mac mini. If it's been 'refurbished' does that wipe out the OS? If so, how would I find/download an older OS like Yosemite etc? This ad has a funny 'screen size' for a Mac mini. 🙂
Depends where you buy it from and what their policy is, I imagine most would clean any data from the drive and then reinstall a fresh copy of the operating system but it will vary by refurbisher. You can always download older version from here https://support.apple.com/en-us/102662 but you will need (another) Mac to be able to do this procedure.
Any mac from the last decade can be connected to the internet and the latest compatible version of mac OS can be installed on it, provided it's been unlocked by the previous owner, or you have the unlock password. No need for another mac to do it.
Most sellers these days knows how to unlock and prepare a mac for ownership transfer. It's very easy. You will most often recieve a mac with the latest compatible OS, and be greeted with the "Hello" screen like it came from the factory. Or you can install macOS again yourself, but it's likely unnessecary.
Refurbished from Apple will be like a new device - except in a plain box.
'Restored' from a Walmart reseller could be anything - and is unlikely to be an M2 Pro with 4 x TB4 ports for $544 as the screenshot suggests; esp. since the 4 x TB4 model had 16GB memory - as far as I can tell from a brief look.
2012 and newer, as @Ben J. said, it's easy to install the latest compatible version by booting with Command-Option-R; erase the storage; "Reinstall macOS"
Prior to 2012 it gets weird.... but if you're buying a 12+ year old machine, hopefully you're prepared for that.
From your screenshot the text says it is a Mini with 8 GB RAM. Probably M1 processor (~4 years old) and, maybe, only 256 GB SSD. I would be looking for 16GB and at least 512GB SSD.
and is unlikely to be an M2 Pro with 4 x TB4 ports for $544 as the screenshot suggests; esp. since the 4 x TB4 model had 16GB memory - as far as I can tell from a brief look.