One note with WiFi only Apple equipment (mainly iPads here); there are ways to use diagnostic cables and software to 'rewrite' the device serial numbers to 'bypass' activation lock.
It requires a 'clean' serial, WiFi MAC, and Bluetooth MAC, the three things a WiFi only device uses to authenticate to Apple during setup.
iPhones and other cellular connected devices? Nah, not gonna happen, though there are weird jailbreak-esque 'bypasses' that will get the phone past setup, albeit still 'locked' when you reset.
I'm sure ebay has their fair share of stolen goods. A lot of what you see sold as refurbished are returns and trade-ins. When you return an item for example, to Amazon, they don't restock it. Instead they throw it in a bin and auction it off by the pallet to second hand resellers.
Are they actually legit (assuming a big seller, not a 20-star account), or are they "laundered" stolen phones?
A lot of them have Chinese ROMs, are they the reverse: stolen in China and sold abroad? Or just arbitrage of regional pricing?