[Dekatron]

Checking other auctions where they sold the same model is the best idea to get a current price figure, although sorting out the highest prices might be a good idea as those normally don't reflect the current going rate (some do but not all).

Check the auctions for if they guarantee that they are calibrated or only sold as spares is also a good idea to get an understanding for the going rate.

I can only recommend that you do a full calibration before selling yours and also that you check the condition of especially the meter, the grid volts potentiometer and the whole gm-wheel assembly and also have a look at all the rotary switches and the roller selector and sockets in the valve panel.