Double legacy Gengar value

My experience that collector's value doesn't amount to much. There might be a legacy collector who really wants it, but most people just want the meta relevant legacys. Rare legacys might command a premium from the person who actually wants them, but the people who want them are almost as rare as the legacy mons themselves.

Someone traded me a Shadow Claw/Dark Pulse Gengar and I TM’ed it to Shadow Ball. It will only be used as a Ghost Attacker. And what would you want back? If I had what you had I would make sure you get a Gengar with SC.

It has the same value as any SC Gengar, which is quite high. If you're looking to trade it, you can ask for almost anything, within reason. Be it a legendary or a shiny.

If from July or August 2016, it is guaranteed to be lucky when traded (if both parties have less than 10 luckies). The other party would probably CTM SW to ShB anyway.