If you want to buy an older used smartphone (produced before 2016), check on which network was the phone initially used. Because if it was used in a CDMA network, it will not work in a GSM one. This is because these two network types are completely different.

So before buying an older smartphone, make sure it was not originally used with networks like Sprint, Verizon, Boost. They won't work with GSM sim cards.

However, the new devices like Samsung Galaxy S8 and S9 support both GSM and CDMA networks.

https://www.unlockunit.com/blog/cdma-networks/