IMEI Null / null / Baseband Unknown topic

Hello there..

Long story short, I bought a Note 3 SM-9005 LTE version from eBay.

The seller said the following:

Before upgrading to Android L 5.0 (Leak) the phone was fine (fully working, SIM working, etc)
After upgrading to Android L 5.0 (Leak) the phone was still fine but.. the SIM card stopped working. Actually, it stopped completely recognizing SIM cards. Also the IMEI turned into null/null and baseband into unknown (something that he didn't tell me.. and of course I found out alone)

Facts: *#1234# = Everything except the modem. It only shows the rest 2 but not the modem.

Also when I saw that the IMEI is null/null I remembered that the IMEI is available behind the battery (there's a label) but guess what.. it was removed..

Conclusions:

A) The phone was stolen by someone (Only the phone came no S-pen, Box, Charger, etc)
B) He indeed messed up with his phone and something got destroyed/corrupted. I don't know.

So I am asking you what can I do right now.. I really need to find a fix otherwise I'm opening a dispute case so I can take my money back.

Thank you in advance.