Hi, welcome to Air Forums. The 03 Airstream you are looking at has the new, latest aluminum coating called Akzo. It is tough stuff. The corrosion this trailer displays appears to be the infamous "filiform" corrosion where moisture migrates along the cut edges of the various aluminum segments.
It can be worked to make it look a little better. And it can be maintained so it won't get any worse. It is not a structural issue, it is a cosmetic issue. Maybe somewhat like the hail damage on my Airstream. It makes the trailer worth less.
You can read all about this filiform corrosion in a massive thread posted under ribs, skins, and rivets category. Folks are angry their new Airstreams display this condition, but folks have developed ways to kinda repair and mitigate it.
More importantly, be sure to check this rig for rotted subfloor at the rear of the trailer. Some trailers have developed leaks at the intersection of the body to the frame and rain water rots the subfloor underneath the floor covering. This condition would make this trailer worth substantially less as the repair is labor intensive.
Bottom line: the filiform corrosion is there and that makes the trailer worth less as it is not cosmetically "perfect". Maybe you like lower price better than "mint" condition. Although the price reduction isn't huge due to this issue.