Kent,
I see it as 2 classes of saws I would buy, one that is in a shape that it needs to be restored, and it is in a condition that it is worth the time involved to restore it. The second type has already been restored and sharpened, The first class is worth $10 in my view, and the 2nd class is worth more than what you say this guy is asking.
Like some of the rest above I have restored old saws, and if the saws are in a condition that makes the worth being restored, IE: straight, aren't pitted, with a handle that is in good condition, the blade has plenty of width left, and it was a good quality old saw when it was new, it ought to be worth $10. That said, I have paid a lot less for very good saws at garage sales.
In my humble opinion, the man doing the restoring and selling the saw has to earn a living, and I don't begrudge paying such a fair price for his labor. To me $25 is a bargain, given the labor involved, for a restored sharp saw.
That said, I would not pay that much for one, because I restore and sharpen my own saws, and frankly have enough in restore able condition that I don't need any more saws. However, if it was a Disston #12 I would certainly pay that much for one. It depends on the saw.
So, I think it depends on a persons situation. If a person either does not want to do the restoring or lacks the time, or is not confident enough to tackle such, but needs a sharp "ready to use saw," $25 is a very reasonable price IMHO. You will have to pay a great deal more than that to buy a new saw that is as good as one of the well restored old Disstons. There are other good brands of vintage saws out there as well.
If you can do all of the restoring, $10 is a reasonable price for a saw that is good enough to be restored, although if you a lucky you can get one for quite a bit less at a garage sale. However, if you need one now, pay $10. Again, I would pay significantly more than that for a Disston #12, in such a condition.
I don't know what it costs to have a saw sharpened in this day and age, it has been probably 15 years since I have paid someone to sharpen one as I now sharpen my own saws. I would guess that it may be $10 or so to have one sharpened now, (wild guess method of cost estimating based on my experience of 15 years ago,) and you need to take that into consideration when considering the $25 figure.
For carpenter size hand saws, I think 3 are a good set, and 4 is better. For the three saw set, I would have an 8 point and a 12 point (or 10 pt but I would prefer the 12) cross cut, one for sizing dimension lumber and one for finish cutting, and in addition I would have one rip saw, and the rip saw could be anything from a 5 point to a 7 point. For the 4 saw set I would have the 8 and 12 point crosscuts, and a 4&1/2 or 5 point rip for ripping dimension lumber or for re-sawing, and a 7 or 8 point rip for finish work. For a single rip, a 6 point, might be a good compromise. Make sure the rip saws don't have a LOT of set when you get one sharpened, commercial guys tend to put more set in rip saws than I like.
Like one of the guys mentioned on another post where such was discussed, this time for planes: which do you have more of, time or money. It will have an affect on you purchasing decisions. Most of my saws were purchase when I definitely was in the more time than money class, and even then I didn't have that much time. Also, you can spend a lot of time looking for old tools that are bargains. If you have time and the skills to sharpen and restore a saw, you can save some nice cash. Like Jim mentioned, there are web instructions for doing both the restoring and sharpening.
Stew