Truth is if we are talking about Mcintosh specifically its not the best value on the planet, but everybody knows that. However, I have learned the hard way to stay away from most gear they make after 2000. Older models are by far a different beast. The newer Amps running between 3500 and 5500 bucks are in fact using many generic designed, with generic part modules.
They do make their own transformers which helps, and they have always been known for their "House" sound due to the Autoformers which most other amp brands don't use. Not bad sounding gear, but yes it is way over priced, over weight, and over hyped due to blue meters for the most part.
As for the used market resale... That's simple as well because most audiophiles know its around almost longer than any boutique brand out there, sorta like Klipsch speakers, and any doctor or lawyer normally buys this brand before any other just like a Rolex watch as this is what they know, and they know it holds a certain prestige. They don't want to sit there and explain what the hell that plain black box is to their buddies hanging out at a cocktail party when they know just sticking a Mac on the shelf will be self explanatory, look good, and make them feel like they made it!
This is not a slam at Mcintosh because truth is they still make some very respectable gear, and you can do far worse even at the money level they occupy.