For clarity, I want to show the facts here - I don't want to make assertions about a person, their experience or other qualities as that is not the issue here.
The advert did lead me to understand the condition was good enough to assemble the mount, perform a few tweaks and then use it without maintenance.
Here and
here are videos of the DEC operating without load. On testing the mount, when I used it to slew this sounded like metal grinding on metal. This may have 'disappeared' under full load, but in my experience if an issue is present, putting a full load on what I suspect to be a failing component would make me negligent and risk the equipment and the mount.
On manually testing the worm, it did not move smoothly but bumped and halted regularly - this was while disengaged from both the motor and the DEC gear.
When I removed the worm block and inspected the bearings, I found them worn and in one case missing a raceway entirely and with dried grease.
Here is the photo of those bearings,
here is a photo of the replacement bearings and
here is a comparison shot.
I have since replaced those bearing, re-greased the DEC gear and worm block, re-seated it and adjusted it under load (evidenced
here).
Here is what it sounds like now. There are no bumps when I manually test the worm drive and no grinding noises when it slews (please excuse the frogs, it's night time).
This leads me to believe that the issue was worn bearings and that this was present when I took ownership of the mount.
As to periodic error - when I get a chance and have correctly polar aligned the mount, I will post the results here. It's cloudy tonight and I'm going away with my family for the weekend, but I'm eager to get this working to specification so will persist.