Condition first
A clean, fully working machine is worth several times more than an identical machine that needs service. Working order is the biggest single factor.
Make and model
Certain models such as Olympia SM series, Hermes 3000, Olivetti Lettera 22 and 32, and early Underwoods command strong demand. Common office standards are often worth less than the cost of a service.
Rarity and features
Unusual typefaces such as script or italic, uncommon colors, and rare keyboard layouts add significant value.
Originality and completeness
Original case, keys, decals, and paint all matter. Repainted machines usually lose value to collectors.
Provenance
Documented history, particularly connection to a known writer or business, can elevate a machine substantially.
For a formal opinion of value for insurance or sale, book an appraisal. For a quick sense of what you have, email photos and the serial number. Related service: Appraisals and Consultation.