I thought the last 550 was the K3 made in '78...Are you sure it's not a 650 ?, they still made those from 1980 - 1982.
Anyway, I agree with most here that's it's likely to be a carbissue. But it's hard to determine if the rest of the bike is any good if you can't really drive it. I would make a low offer to be on the safe side.
Also check for any missing parts, so you won't have spend a lot of money on new parts.
If it's a '78 550K3, or any other 550 with 4 pipes, take a real good look at the underside off the mufflers and headers. A lot of these are rusted really bad with holes in them, this comes from condensation that builds in the pipes (short rides).
These chrome 4-4 pipes are extremely expensive to replace, and a permanent fix is also difficult.
Other 550 parts that can be expensive or hard to find are: Front- and Rearfenders, lamp-ears, forktubes, sidecovers and tanks. So make sure these are ok.
