Nissan MR20De - two spark plugs with broken electrodes.

I recently asked for assistance on diagnosing my 2006 Nissan Serena which has eventually developed enough of a misfire to establish it was a spark
issue. Since I am at a different job and don't have access to a workshop I took it to the dealers today - the intake manifold had to come off to get the
spark plugs out. I had thought we would be replacing ignition coils so I bought 4 and had them ready, along with a new set of plugs.
Now when this issue started the guy who sold it to me told me he has fitted new plugs 10 000km ago.
I believe he is honest, he has been very up front about the deal.
After contacting him and telling him two of the plugs no longer have electrodes he said he got the set of plugs brand new but they were cheap. They could
possibly be imitations. I'm wondering id anyone having a look at this picture could suggest another possibility - system running too lean perhaps?
Or possibly just rubbish plugs?

 

The slight brown coloration on the end of the insulator, I have always taken as a sign of complete combustion.
Lean usually leave white deposits and rich black.
They just look plain old used up in my opinion.
I'm interested in seeing if anyone else can glean more information.

Thanks for the input - someone has pointed out to me that the two bad plugs look a bit different in that the insulator is thicker.