I just looked at this 79 GS850G this afternoon. It's not near as clean as the 80 model you're looking at.
Guy wants $350.00 for it. Fairly new looking Dunlop tires and a newer battery. Appears to be pretty much unmolested. Electric starter doesn't work. Not even a peep out of it. Maybe a relay? Over all it doesn't look too bad for $350.00. Needs a good clean up and has 17k miles.
Carbs need attention. Float(s?) sticking and it dumped some gas out when I turned the fuel on.
I'm gonna sleep on it but think I'll probably take it. Since I can't hear it run I think I'll check the compression prior to any $$$$ changing hands.