If you are going to buy secondhand phone you have to check its IMEI number, and that becouse IMEI is used to identify mobile devices. Using IMEI you can find out exact information about your phone, and also you can check its blacklist status.
You want to know your IMEI number? Here you will find a few methods of finding it.
Using the Dial Pad Open dialing app and type *#06# in the dialing field.
In the phone menu Go to Settings and then System -> About phone -> IMEI
On the box You can find IMEI number on the original box of your phone.
On the SIM tray If you remove SIM cards you should see IMEI number labeled on SIM tray of your phone.
Here you can find a video tutorial on finding IMEI number on Google Pixel XL 2