For drivetrain components, I only buy NOS (and only when similarly priced to equivalent modern units). I don't see the point of getting some heavily used chainrings or knackered shifters just because they were the ones the bike used to have when new. For other components I don't mind used if it's reflected in the price, but I never pay premium for "vintage". If I need new brakes and some LX cantilevers cost the same as new Deore v-brakes, as it happened recently, I won't hesitate to get the new stuff.
Usually, best value purchases come from sellers that don't know what NOS means. They are just selling that old bike that has been gathering dust in the shed and as such, they sell it for what it is, an old bike. Luckily where I live now, in Germany, it is relatively easy to find decent deals as many people have big sheds with too many bikes lying around. But whenever I see in the add words like "retro", "rare", "vintage" or "highly sought after", I tend to run away, as the owner has probably a (at least to me) totally unrealistic price.
Also, forums like this one where all of us fanatics are concentrated are great to find rare pieces, but not the best of prices, as we all think our "old sh*t" is worth more than it really is...
My mother-in-law still wonders why I bought that used 25 year-old lugged steel Trek 950 for my wife, when "it has no suspension and the handlebar is so low...". For her, and she's probably right, it's just an old bike. At least I got it at a decent price!
