Aneroid barometers that have not been properly serviced and calibrated may be prone to one or more of the many common faults that are found and the oft-cited Plastic Bag Text is fallible. Buying an instrument that has not been properly checked is therefore a lottery.
An original Negretti & Zambra leaflet c1910 with directions for setting up self-recording barometers (barographs), thermometers (thermographs), etc
The decline in the antiques trade might be a wholly unnatural phenomenon. Is it about to rebound with a rise in the collection of technology?