Sofa Compact found  

You're approaching it all wrong...
No need to keep him in the dark. After all, he's probably doing OK financially as you said. If you were to tell him that you really dig the sofa and would love to take it off him as a favorish sort of thing you'd feel much better about it. He'd end up replacing it with a POS anyway. You could even offer to replace it with a thrift store find or something.

Depends on what kind of doctor.
General practitioners don't make a lot of money contrary to popular assumptions. And considering what they have to do to become doctors, their income level is not that great. That is why so many medical students are deciding to become "specialists" and good family doctors are becoming harder to find.
I say if the doctor is a G.P. either offer him a fair price or leave it in his office. If it's a plastic surgeon, bring a couple friends, create a distraction, grab the sofa and run out the door!

"Found"?
I'm not sure what you mean; if you came over to my house, you'd "find" a Sofa Compact, too, but that doesn't mean that it'd be for sale.
Is that the case with your doctor's sofa, too? If so, then I sorta doubt that he'd want to sell it at all... I mean, he needs a sofa in his waiting room, and the one he's got is already doing the job for him; unless you somehow make it easy for him to sell his sofa to you and get a replacement immediately, I don't see why he'd be interested.
Regardless, though... If you want to be scrupulously fair, treat him the way you'd want your mother to be treated by someone interested in buying HER sofa.
Tell him, "Wow, I really like that couch, Doc. If you're ever interested in selling it, let me know. Seriously."
If he tells you he wants to sell it -- either right then or a couple years later -- ask how much he wants for it. If he gives you a figure you like, take it. If it's more than you want to spend, thank him but say that you really can't afford to spend more than X dollars. If he takes that offer, cool. Either way, you're done.
It's tougher if he says that he has no idea what it's worth. If you want to be fair, you have to tell him its value. You're right; his high income doesn't relieve you of that responsibility.
Tell him that if it were of recent vintage and in perfect condition, it could sell at auction for $1500-$2000, but reupholstering the one in his waiting room would cost $500-$1000, plus this isn't an auction... And besides, you can only afford to pay $X for it anyway.
At that point, it's up to him to either agree to your price or not. Here's where his (presumed) high income can help you: If he makes a ton of money, he might be more willing to agree to your low offer than a lower-income seller would be.
Good luck...

GPs do pretty well...
At least those in my family do. They're not necessarily making it rain or anything. But I'd guess my uncle would respond with, "Sure, whatever, just don't replace it with anything that'll get me sued" if presented the same scenario.
Heck, offer him a lower tier but brand new sofa to trade. New for old...

a possible solution
My solution is to ask him what he wants for the sofa. This legally clears you from any prior knowledge you may have about the true value as the purchase price is his idea.
Also a doctor is no different than a WalMart employee ( with the exception of a possible greater education), they are all service people.