Personally, i dont think you should be doing anything to the IMEI at this point. Obviously someone changed something along the way. Since both IMEI's appear to be good, theres a good chance someone replaced the motherboard at some point. If this is the case, write the IMEI down! TWICE! Keep it safe where you will not lose it.
If it was a motherboard replacement, DO NOT try to change or otherwise mess with the IMEI! Replacing it with anything but what belongs to that board is asking for problems.
If the mobo is original and the IMEI was changed for some reason, it might be a good idea to change back....maybe! But this doesnt make any sense if the original is still good. If you do somehow verify it was changed, i would wait before doing anything. Someone could wait another month or two and report it lost or stolen. It happens all the time! And then the IMEI would get blacklisted.
I think you should contact the seller and find out what is really going on. But regardless of the answers you may or may not get, do not change anything yet.
If its working fine right now, just leave it as it is. Messing with the NV Data is not to be taken lightly. Its not simple stuff like flashing roms. Make a mistake and it could cause irreparable harm to the device.
---------- Post added at 03:50 AM ---------- Previous post was at 03:47 AM ----------
Oh, and there are several blacklists out there. Just because it isnt on one, doesnt mean it cant be on another. Check on swappa, but definately call the carrier the device originally came from. Ask if it is able to be activated on any network including theirs.