Just remember the words of that famous old Huckster, P.T.Barnum! "There's a sucker born every minute!"
My take on it: If you are a good mechanic, and know how to fix lawn and garden equipment, and engines, and other things---well--yeah, you might luck out!
But--what happens when yer "good deal" goes sour, with a blown engine a month after you have bought it?!
Did he have a maintenance plan to keep tabs on oil levels, tires, blades, belts, all of that and more?
I think you'd be buying a "Pig in a Poke." My experience with those deals is that the buyer gets the shaft, in most cases. unless you have known that guy all yer life, and know how he ran his business, and had a maintenance plan for his equipment, i'd say:"Stay away!"
If he's just trying to get rid of the old stuff, make him an offer. I have bought up some things by making a "ridiculous offer"! Caveat emptor!

There's pretty much 2 reasons why a commercial entity will upgrade.
1. The stuff is worn out.
2. The stuff is almost worn out and repair costs/down time is no longer economically feasible.

well I finally got it the other day,
SEEMS to work great, lots of maintenance records since new,
Large in-house maintenance shop LOTS of nice equipment.
rough in appearance tho.. functionally working good.

but I will keep in mind what you said.... any thing cvan happen anytime

Well, maybe you lucked out! Hope it works for you.
A friend asked me to look at one of those big mowers, the commercial walk behind type. I made it run, and did some other work on it. I told him not to allow his wife to use it, as it was not for the "faint of heart" to try to run it!
So, one day while he was at work, his wife got it running, and finally figured out how to put it in gear!
And thats when she discovered why he didn't want her using it. She went straight thru their vegetable garden, mowing a large, wiggly path thru it, and finally let go of the controls, while the machine went off a high cliff behind the garden, at speed! Needless to say--he sold it right soon!

WOW that's a mower story!!
being that they weight nearly 130lbs, with solid dry traction and being in high gear, I could see them "pulling" an operator along at 4.5 mph.. hehe

I myself only weigh a buck fitty, and have a bad back (why i got the self propelled model)

i KNOW its over kill for my smallish 8000 sq ft yard, but its Very rough, more field like than Yard like.. and I just Love having the toughest thing out there that will last the rest of my mowing years..

This SHOULD fit the bill... Im actually finding myself restoring and fixing it up (unplanned) to look like the 1200 machine it was when new!!

To each his own. 8000 square feet , could be very well managed with a lot less mower. My yard, has about 30,000 sq ft of grass. I dasn't call it a lawn . I keep it short with the remains of an old IH sit down and a old Toro 3 speed walk behind. I am also in the process of getting a vintage Gravely walk behind going. It has a sickle bar on it. The oil bath air filter says Studebaker .