How to estimate the value of a watch?

How to tell whether a watch is in new, excellent, good or average condition.

Never been worn. No visible scratches, no trace of impact. Original bracelet and clasp as new.
Exceptions: none.

Excellent condition : Worn rarely, all original parts, in perfect condition.
Acceptable: A very few, fine and tiny scratches, slight marking of original bracelet.

Good condition : Worn well, perfect working order; crystal, case and winding crown cleaned and serviced regularly, bracelet with original clasp in good condition.
Acceptable: Small, barely visible scratches on case or metal bracelet requiring very slight polishing (or change if leather strap).

Average condition : Worn frequently, not well-maintained, not serviced during previous 5 years, service required, need to check water resistance, case requires complete polish, some work required on mechanism, leather strap replacement essential.
Acceptable: Numerous small-to-larger scratches visible, possible problem with mechanism, crystal scratched, poor anti-reflection finish, bracelet worn out.