You have to have them tested, calibrated and certified at an official cal-lab .
The other issue is - what exactly do you have ?! I have mostly dealt with industrial controls and instrumentation, and re-sellability is all based on what you have, the name brand, the condition, and the calibration certs !
( Example ) If you have a certified Rosemont differential pressure transmitter with Hart, I would have no issues paying $600 per unit .... on the other hand if you had an uncertifyed 'no name', I would not pay more than scrap.
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