If it doesn't work you should be able to return it under the ebay buying policy unless the seller said it was being sold for repair or parts.
One other thing, I'm a big fan of the IQ cams as well. One of my outdoor cams failed about a year ago and I ended up replacing it with the regular Gen 1 Nest Outdoor (the one without the "IQ"). It saved a lot of money and I don't see a signifiant enough difference that I regret not getting the IQ. That being said it is located in a area where I do not need to see great distance.
As far as adding it to your system as long as the previous owner "REMOVED" it from their nest app you should be good to go. If you get an error when you try and add it that says something like "already registered to another account" then you will have to contact them and ask them to launch their app and choose the REMOVE action so it is no longer registered to them.
Another hit is because of the way they designed the app, when you go to actually ADD IT to your system you will want the new camera to be near (abut 10 feet or so) another NEST device you already have. This can be a camera or a smoke detector. The reason for this (as best I can tell) is the two nest devices do a sort of handshake via Bluetooth to transfer information.
Here is a link to the official instructions for adding a camera. It may be worth looking at before you start the process.
https://support.google.com/googlenest/answer/9293657?hl=en
Hope this helps and let me know how it goes.
