iPhone 5 IMEI blacklist?


I buy repair and sell phones and often refurbish them.
I bought a Note 4 the other day and it is not blocked (first time ever)

A customer bought a phone a month ago and is it now blocked, yay…

Is there anything at all I can do or are they now useless in the UK?
Also, can anybody recommend a good site (ideally free) to check IMEI numbers accurately? I use IMEIPro at the moment

Thanks a lot guys, any advice is greatly appreciated as I am sick and tired of people ripping me off.