A famous stamp design, an old date, or a large internet asking price does not establish value.
The value stack
Use a narrow comparison set
Compare completed sales of the same recognized catalog identity and similar condition. Separate mint never hinged, hinged, no gum, used, on-cover, faulty, repaired, uncertified, certified, and numerically graded examples.
Why catalog value differs from cash value
Catalog values are reference figures produced under stated assumptions. They are not universal offers to buy, and common material often trades at a fraction of catalog while exceptional certified examples can behave differently.
Inherited collection expectations
Most collections contain large quantities of common material. Value may be concentrated in a few certified stamps, postal-history items, specialized studies, varieties, or intact collections with provenance.
Compare current collector listings
Use listings to study descriptions, scans, certificates, and market supply. Asking prices are not completed-sale evidence.
Compare collector listings