they are quality bikes to work on, handle better(although heavier) than a cb750, much more power, Mikuni carbs seem better to me too...do some research on what model gs1000 it is, I think there is a "g", a "e", "L"?...one of which is the ugly cruiser style which is hard to give away, sportier ones and standards seem to go for good money...
If I were you, I'd find out what model it is...if its the cruiser? walk...if not waive $300 in his face and tell him it's really just a basket case after all.