Good luck "Selling the Manual" because they mostly want to sell new equipment products that can "Hopefully" last the warranty period and then the seller is is off the hook and both the retailer and manufacturer can hope that after the warranty most end users will "buy new" rather than fix old products especially when the end user has no "repair or parts information"
Yep, I'm getting a new furnace this summer... but from the same outfit I've dealt with for 40 years. I'll actually give the techs the old one's manual. They'll part the old box out as time goes on. A lot of people around here make do and make stuff out of practically nothing because they have almost nothing... the last time I had a radio antenna ripped off a VW bug parked on the street in the city, my bro-in-law made me one out of a piece of the brake line of a 1938 Dodge truck still parked outside one of his barns, think it belonged to his dad. When he finished the repair job on my car he said "this antenna here now has the additional advantage of looking so bad no one will steal it." Local mechanics and appliance techs are used to that whole backlot inventory mindset and so they do keep parts that are still good off old things.
Gas (propane) furnaces are pretty long lived past their warranty dates anyway. I just don't like thinking about having to wait on a repair of mine in winter again, since I no longer use backup woodstove heating. Thing is 17 years old on a 10 yr warranty so it's time to move it out. Looking forward to newer model with the more efficient combustion, not even needing to vent via chimney, just outside through a cellar window like a clothes dryer.