Take care when buy a used cell phone.
6000 phones are reported lost or stolen PER MONTH and blocked by the phone companies here
.http://www.elpais.com.uy/informacion/mes-bloquean-celulares-denuncias-robo.html
This articles tells you how to check a phone to see if it has been blocked. It can take a few days to show up on the list though. Asking for proof of ownership before purchasing a used phone is wise.