NavList:

Often times, these things are inherited by a spouse, or descendent, who cannot accurately assess their condition, or it's been sitting in the bilge for five years.

Second - look for the asking price for equivalent models and decide both on your price point and the possibility of the cost of repairs.

As others said, when you bid, set the alarm clock and hit the button with seconds to spare.  

I went for a relatively low-asking cost Tamaya.  It had been sitting in the bilge for years.   Fortunately the main pieces were in good condition.    The mirrors had to be redone, and the drum mechanism needed some sprucing up.    My local outfit was able to do the repairs, calibrate so that the index correction was nil.   I suspect that the cost of the sextant+cost of repairs might be almost an invariant in a 'fair' world - that is to say, the lower the asking Ebay cost, the more repairs are needed, but that's just a guess.   I decided to err on the cheap side of asking and trust in my repair shop, who I knew was good.   

The box was in bad shape, as you might imagine, but I had a relative who meticulously restored it.   I finally fashioned my own handle for the box.