I saw that one for sale and it's not quite what it seems. It is advertised as a '75 model (GPI) but it doesn't appear to have a GPI frame/swingarm. the GPI has laydown shocks and the top mount is further forward than as pictured on the advertised bike. The whole frame in that area looks wrong, for a GPI. It seems to be a '74 model frame, with the shock mounts on the swingarm moved forward a bit. Not sure whether it is a '74 aluminium swingarm (painted black) with mounts moved forward or a GPI CrMo swingarm with moved forward mounts.
Similarly, it is strange that he's saying that the bike has GPI gearbox updates, as it wouldn't need them if it was a GPI, if you get my drift. Also, the fork legs appear to be '74 not '75 (some, not all, '75 models had a rib on the forkleg).
Regards
Ken