There's a loaded question.
Some of those are old enough you can almost get away with "vintage". If I had one to sell I would explain that it's not quite the same as the truly old ones though.
Is it in running condition? What do you plan to use it for? It should be a low shank machine, use class 15 bobbins and regular 15x1 needles. If you want to darn or FMQ, a hopping foot is good to have. Do you have (or need) a buttonholer and set of feet for it? Ebay is also a good bet for feet. Extra bobbins are always nice and Sew-Classic sells a far better quality bobbin than the Singer branded blister packs at places like Walmart.
Hope this helps,
Rodney