I'd say marts' suggestion might be best , regarding reselling such an item .
A&P mechanics might want to have such an item , but for display only .
These types of tachometers are regarded as unsafe , in modern times , as it requires a mechanic to actually hand-hold the tach & make the tool come into physical contact with spinning (& sometimes dangerous) shafts , sheaves & such .
Such tools are required to be regularly inspected & certfied , in the event someone wishes to use them on ANY aircraft application !
If indeed you intend to resell this item , I'd strongly suggest that you advertise it as a 'wall-hanger' (so to speak) only .
The parts within these mechanical tachs do wear & replacement parts are rare as can be .
After all that being said , I think it's a cool little surplus (non-antique) item , beagler15 !
