Mike C - thanks for asking your deck question. It's something I like talking about so lets do a little analysis. First off I'm not really disagreeing with Charlie, Kraig, Nic or Ron - I just look at it a little differently. I do agree with Nic that it's only worth what you're willing to pay. But what should you be willing to pay is the question.
When I look at this deck I don't see the real working side. What I do see is the mostly cosmetic side and it does look like it's been used but probably well greased. I see the idler pulley and belt but I don't see a hole in the cover to grease the idler bolt bushing. I see flat runners on the sides - but wait, I'm not sure a 42" had flat runners. I was thinking only the 38" got flat runners. Maybe they are replacements for the bent rod runners - anyway, they look fairly good which is a plus. I think there was a late series of 42" decks (before the 44A deck came out) that didn't have gauge wheels). I also notice the 2 yellow spindle covers and they don't appear rusty but one seems to be missing the front part around the front bolt hole - it could have broken off or rotted away and tore off. These covers usually rust out the bolt holes and end up being removed so having them in place is nice. Then I see what may be a patch on the front edge face (on the side below the IH decal). That may not be a good sign and makes me wonder what the condition is like on the underneath side. Is the blade/grass guide "W" shape in place or missing? Is the underside clean so you can see if it's all pitted up? Are there any cracks in the deck skin, especially around the rear hanger brackets, the holes where the guide runners mounts, and especially where the gauge wheels mount? (Wait - it doesn't have gauge wheels. Is it supposed to or is it a later 42 that came without them?) What condition are the spindles and blades really in. You really have to remove that belt on the spindles to see if the spindles are tight. I've been fooled a few times. If that belt is nice and snug (has a good idler pulley spring) you might not be able to tell if the spindle bearings are loose a little. And the blades - are they down to their last sharpening - which is why he has an extra set - or is the extra set ready for the scrap pile?
All the things I mentioned can be costly new replacement items - which I also think why Charlie's seeing market prices of $50-$75 for decks that look better. Nic gives you a method to analyze whether it's worth $150 to you, and it sounds reasonable. Ron has a non-QA for alot less and it could probably work but Ron isn't close to you and shipping could be alot to NY.
So overall it comes down to prices in your area in NY and I believe NY usually commands higher prices for good used equipment, higher than you see in the midwest. It comes down to what's available in your area, and reasonable driving distant, and what condition this deck is really in, and how much you really need it.
It used to be hard to find good used decks. Most people sell their tractors with a deck. If they separate the deck it's usually harder to sell the tractor especially in the spring which is prime season for L&G equipment and prime prices. 20 years ago I'd see an occassional deck for $100 and maybe just maybe up to $150 but never thought they sold for that much. Why would someone sell a deck by itself. They need it for the tractor - unless the tractor blew up. Today that same deck is 20 years older making it 40 years old. Is it worth $100-$150. All depends on condition - but you better have a good close look at it. If it seems as good as the guy describes to you, tight spindles, good idler pulley and belt, good blades (not wore to last shapening) well it might just be worth $150. You don't live in the Midwest where Charlie and others see so many of these tractors and lots of decks available. Where you are you may only see 1 or 2 decks a year. If it's like where I lived in CT, I would see 2 or 3 decks when I didn't need one, and see NONE when I did need one. I used to drive to NJ or PA to get stuff - just because it was available and was a reasonable price, but gas was $2 (wait it's about $2.50 now so it could be worth a drive but not if it's junk when you get there). I actually don't recall ever buying just a deck. As this developed into a hobby for me I learned to buy a whole tractor more often if I needed something, and usually use the rest of the stuff on the next tractor I looked for or got. It just made more dollars and sense to me.
So what would I do in your situation? I'll assume you don't want to buy a whole tractor with blown engine and good deck. In which case I'd have a look at this deck, and if it's decent I'd offer $100. I'd just be straight up with the guy that I really don't need the deck, that I'm fixing mine and it will be fine after I fix it, but if I could find a decent used deck for $100 I'd like to have one, and still fix mine and keep it as a spare. And by the way, where is the sub-frame? It only works with the mowing deck - no good by itself for anything else - so it should go with the deck for the same $100.
Good luck and let us know what you end up doing.