I often find a very slight haze in lenses that are even just a few years old; plus it often returns every couple years. I've learned to work on my own as a way to keep this from driving me crazy. The faint haze I'm thinking of is hard to see, so I assume most sellers will just not notice it. The pro dealers use a small flashlight to check, especially when they are buying :-)
But it is far too common to buy lenses that have obvious haze, fungus and other problems. It is certainly frustrating, and expensive. Nikkors are the least prone to these problems of any lenses I've seen, though obviously stuff can happen on anything 40 and 50 years old.
The Seneca lens linked above should be a fun project. It looks like a rapid rectilinear based triple convertible and looks to have Uniform System aperture markings. Hard to read the focal lengths. Is it 6"/14"/18". I hope we'll be seeing some great photos from it and the Olys.