thenurse
First check it's the right size for you then check it has all its bits pieces (replacing can be costly)
I think there's a HUGE difference beween a few scratches/scuff/dirt and structural damage.
I wouldn't buy a house that I couldn't live in and I wouldn't buy a bag that I couldn't use. If it's just a question of cosmetic repair that spa treatment could cover, fair enough. If it's cracking leather past the point of no return or torn leather etc then I wouldn't even make an offer.
Therefore there are 3 price points
1. Sound bag but aesthetically challanged - offer what you can comfortably afford but keep some Norwegian Krone back for a spa treatment
2. Sound bag, needs TLC and bits missing (like shoulder stap etc) - offer as above but minus what ever it costs to replace missing parts
3. Bag that has any leather torn, deeply cracked, handle broken, perished leather etc I would not make an offer.
The bag in the pick looks like 1 or 2 but I would ask the seller to outline exactly what they mean (from what you say it sounds so vague).
Like any bag, outline what you think is acceptable and what is not before you pay and make it clear that you will not accept/return a bag that crosses the line between.
The actual figure that you pay the seller is what ever you think it's worth and what the seller will accept.
Good luck!