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That disconnected cable is probably for a light to read the scales in twilight. It looks to have a simple electrical "plug" that probably goes into the round brass socket at the top of the handle. Batteries would be in the handle itself. Look inside to be sure they haven't been left in there to leak Also there must be a push button momentary switch somewhere on the handle. Check that it too is in good condition. The illumination circuits generally are not critical and if a little damaged do not require special skills or equipment to repair.
When shopping for a used quality metal sextant make sure it has its certificate. Every owner of a quality metal sextant knows the importance of keeping the certificate so if it is missing one must ask why? Carelessness? Major repair? etc.. A quality metal sextant without a certificate might be OK, but it just might not be, and it is quite a chore to check it out. And once you do you still have a sextant with no certificate which will seriously effect its resale value down the road. Plastic sextants do not have certificates. Replica sextants usually don't have certificates either, but it would not be difficult to fake one. BTW many certificates these days simply say something along the lines of "Instrument is free of errors for practical navigation" Not very helpful, but still it must be with the instrument.
Next check that the optics are clear. It takes a real pro to clean them internally if it can be done at all, and a little cloudiness or mold will seriously effect your usefulness in low light conditions (twilight.)
Then it is on to the mirrors. They can be replaced but it isn't totally simple and just like the scope if they are showing corrosion it effects twilight use.
Shades must move easily but not flop. A stiff shade stack or one that will not stay put will drive you crazy. It often looks as if you can just tighten a pivot screw to fix this but usually that is not the case. Often those screws have pins in them to stop the from coming loose on their own. You can push the pin out and adjust the screw but only in half turn increments. Generally one way is too loose, the other too tight. Also there are often spring washers in that assebly and if those need replacement it will be a good trick to get one that is just right.
The index shades have to be dark enough to view the sun safely. Look through them at the sun by looking over or around the scope, not through it. The magnification of the scope makes looking at an improperly shaded sun VERY dangerous. The shaded image of the sun (not looking through the scope) should be darker than you think necessary because for sun sights you will be taking many sights in a row, not just a brief peek. Two crossed polarized shades are generally inadequate as sun shades. I would personally avoid such set ups.
The index quick release should work smoothly and the index arm should move smoothly through its range without any rattle or slop. When it is engaged on the ark try giving the index arm a gently nudge in the direction of its normal motion. It should not move.
Make sure the thimble (knob on the micrometer drum) is secure and not slipping or mis-indexed Also be sure the index marker and vernier (if it has one) is present and undamaged.
The handle must be secure. Holding it firmly give the sextant a gentle shake to confirm it is fully secure.
These are just a few things to check out, and not every sextant is alike, but if the instrumet is being sold by a knowledgable and reputable dealer and this all checks it is probably pretty good..
If it is being sold "as is" by someone who says they know nothing about sextants but has it priced as if it is in perfect condition take a pass. That seller has obviously learned enough about sextants to set the price but won't stand behind his sale. Hmmmm...
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Just check the handle's battery chamber- it may be corroded from bursting batteries.
If it "feels" smooth in movement when turning the mirrors and index arm and there's no obvious damage- I'd buy it for $300.
You will have to adjust it, but that's part of using any sextant.
