What is the scrap value for a 777-200ER, what is its used parts value?
The untraceable used parts value for a 777-200ER is... ZERO. Used and reconditioned parts can be installed on other airplanes, but not without Certificate of Conformity / Form1 and so on. There is a lot of paperwork involved in getting a spare from one airplane onto another one. This includes full traceability. Without this paper trail, the part is useless. And faking a paper trail is possible, but doing so for all parts of a 777-200ER is beyond what's possible without raising red flags.
If it were a old 737, a DC-8 or a Cessna, It could be plausible. The people exploiting some old aircraft in some region of the world live under a, let say, different regulatory oversight. But I doubt any 777-200ER operate under conditions where you could use bootlegged market parts. You may as well sell the raw materials.
I believe that a much better reason to make an airplane AND its passenger disappear, is its payload.