OSCA

As you can see the frame is very ratty. Here are some of the things worth mentioning. None of them make the frame unuseable.
The seat cluster is the roughest part. From the looks of it, it appears to have been heated up probably to remove a stuck seatpost. From the pictures you can see welding marks on both sides, I don't know why they are there, however I’ve seen a small hole there on other air frames, again I don't know why. As you can see in the last pic, there appears to be a crack running through the extra welded area. It doesn’t go through the tube. I’m fairly sure that it is fine and it has no structural effect.
There are also similar welding marks in that position near the bottom of the seatpost. When got the frame I had to sand it down so it would fit in the frame. Because you aren't able to twist the post when you insert and remove like a normal seatpost, it requires a bit of force to get it to budge.
The seatpost head is a bit of a bodge job, basically a nut at the top which tightens it all up. It has a habit of slipping unless you do it really tight. I've been told applying a bit of carbon paste would sort that out though.
One last point, the left side rear dropout is a bit of a pain to fit a wheel into. It appears to narrow slightly meaning can be a bit tricky fitting a wheel straight. You probably just need to take a file to it and take some of the material away and it'll be fine.
If all of that hasn't put you off then the bike is all yours! It is really an amazing frame, rides fantastically, great fun. You're unlikely to see another one come up again quite like it. Columbus air is a beautiful tube set.