I opened it to check a full compliment of tubes and powered it up open to check that I could see the heater coil glowing nicely and they were in all the cases I could readily see the heater. I could not see any visual damage to the tubes or any other components.

Here are the tests I carried out and what happened (yes... I'm an old-ish test engineer by profession, just not a tube electronics expert)

Apply power: Power light comes on, measured 6.9V AC at the 6.3V AC terminal, a few hundred millivolts at the high voltage terminals.

Select Standby -> On: still 6.9V at 6.3V AC terminal, DC voltage varies under control of the voltage knob from -7V to approx 430V, with good tie up between the meter and my Fluke DVM. I'll declare the meter and its associated DC voltage circuitry to be working well. One thing I noted was that the voltage control is probably logarithmic, it's certainly a long way off linear, but it is steady when set using the large meter or your DVM.

All above done no load, so meter selected to volts.

One test done with a 15k resistor across the DC output terminals to check current metering. Again, a nice steady current on the meter and on my DVM with the meter under-reading what Ohms Law and my Fluke said the current was, about 20%, but as I was operating in the zero to 20 mA range given a relatively low wattage resistor that I had to hand, that's a visual estimate, with three data points at 50, 100 and 200V.

So I am going to sell this item as functional as described above. Given the nature of vintage electronics containing tubes, there is the risk of things happening en route. 

It will be packed well and.......

I AM GOING TO PACK IT INTERNALLY TO MINIMIZE THE RISK OF TUBES RATTLING OUT IN TRANSIT AND OF COURSE YOU WILL NEED TO REMOVE THAT BEFORE USE. I WILL INDICATE THIS CLEARLY ON MASKING TAPE STUCK OVER THE SWITCHES.