Fun with Android...
I recently bought a phone from Ebay (boxed and complete with all original accessories, so not freshly acquired by a teenager on a moped), only to discover that there's a new security feature on some Android phones: after a factory reset, you have to log in with the old account to verify that the phone hasn't been stolen.
Sadly, the seller had done the factory reset, but hadn't followed up with the verification, and completely failed to respond to my queries; I ended up raising a refund/return request, and still only got an automated "you can return it now" response on the very last day before Ebay would have auto-refunded me.
Thankfully, a bit of searching around threw up a solution. I can't remember the exact details, but it was something along the lines of: open the keyboard's accessibility options and click through things until you got to a help page where you could trigger the Youtube app, from which you could get into the phone's settings and trigger a full credential reset.
Then it turned out that the phone was locked to the wrong network. Fortunately, there's people selling unlock codes on Ebay for 99p, so I just bought one of those - far cheaper than the high street or dedicated unlock websites, and I'd pretty much given up on trying to get anything else at all from the seller!