If you looking to purchase a new smartphone and are gonna be buying It on amazon or ebay then you should first ask the seller if they can give you the imei to verify it is not backlisted.
If it is backlisted than you will not be able to use it and you will essentially have a paper weight sitting around...
The best place to check your imei is with checkesnfree.com
From there you will get the information you need quickly so you can go a head and purchase that expensive smartphone without any worries about being able to use it.
You can do a swappa imei check as well and there is more information about that online as well.