The value of a vintage airline poster depends on a lot of details, condition is just one of them. An important criteria is the age of a poster. And the name of the artist as well as the airline or destination which is depicted. The very first examples of a new style (e.g. modernism) are mostly expensive too.
Another criteria is definitely, how expensive it has been sold at international auctions before. For your interest, I publish every month a few selected auction results:
Auction results 2018
Auction results 2017
Auction results 2016
Auction results 2015
The condition of a poster can be improved by linenbacking. This is a poster conservation method, popular in many countries (e.g. United States, Australia, France).
Museums and a lot of European collectors prefer backing with thin acid free Japanese lining paper only. If a poster has been restored by professionals, in nearly all cases it has been backed too.
Different sellers have different definitions of grading condition. Make sure you understand what is meant by „fine” condition. Ask about folds, tears, stains, scratches, pin holes, foxing, dirt, discoloration, yellowing, fading or any retouches or repairs. Make sure you know the condition of the poster before it was backed and how much restoration has been done to it.
The criteria under which vintage posters are evaluated, is different from that of printed graphic art. A vintage poster can still be classified as ”very good”, although slight creases, stains or minor tears may exist. If a poster has been restored by professional, that should be detailed in the description.
Never buy dry mounted vintage posters! Dry mounting causes hyperacidity of the paper, after a while the paper yellows more an more. This process cannot be reversed and devalues the poster.
If a poster is backed on pasteboard, foamboard, masonite or similar material it is worthless too. In most cases it is impossible to detach it without destroying the paper.
A+: Mint / Perfect
The poster presents absolutely no defect. It’s colors are very fresh, the backside is clean too. Very rare.
A : Very fine
The poster can present very light traces of wears, some light marks of folds, not more than one small pinhole in each corner.
The poster can present traces of wears, light brown spots, some marks of folds, some light marks of tears, more than four pinholes. There may be some slight dirt or very minor restoration. A poster that was folded but is otherwise flawless is rated as fine.
B+: Fairly good
The poster may have traces of wears, spots brown, marks of folds, tears in the margins or the surface of the poster, little down colors. Possibly with minor light-staining. It can show traces of minor restoration.
B : Average
The poster can present some slight paper losses, brown spots or other, folds, tears in the margins or the surface. The paper may have yellowed, the colors may be light-stained. It can show traces of restoration.
B-: Slightly average down
The poster can have paper losses in the margins or the surface, many brown spots or other, marks of folds, long tears, insolated colors. It can show traces of major or noticeable restoration.
C: Average down
Light-staining of the poster may be very pronounced; restoration, folds, or flaking are clearly visible. Only collectable if very rare or old and sought-after. Needs restoration.
D: Bad condition
Many parts of such poster may be missing or the colors could be extremely faded. Not collectable.