Not all 'hoarders' are the same in every respect.
Some guys are just super comfortable having all of it. Although a lot of smaller junk yards have gone away, they feel at home surrounded by it all just the way it is.
In some cases it may be as simple as companionship. Although they won't sell to a stranger, they might to someone that they feel a connection with.
Ding ding
I have been labeled a hoarder before because I have a lot of cars round back, I mean quite a few. I didn't do it on purpose but my family used to be in the salvage business and when the business went down I grabbed what I could because if not then it would go to crush because when a salvage yard goes out of business and the land is sold then everything goes, even the office building in this case.
My cousin came along and added to the pile and now we have two peoples collections going in one barn and back yard which is a nitemare. Most of it I save just because its cool. what's cooler than a cool old mopar? TWO old mopars!!!
Best advice to approach the person with a lot of spare parts. Be honest and drive your old car there with you. When I see a cat show up asking for parts in a new Toyota, my first thought is "here is a another goddamn flipper trying to get rich on my ****" (where is the shot gun)
When a guy shows up in his car then I know he is a car guy because he is driving his car like a real car guy and it makes me want to dig through the pile to see what we can bolt to his car.
Its not about the money, its about completing the puzzle and a car that needs odds and ends is a fun thing to a "hoarder" That is why we save everything because you never know when that odd clip, bolt, or small piece, will complete another puzzle.
I have given away a lot of stuff like that before but I do know the value of certain things so its a balance.
Talking and to the owners and being genuine is the key and don't make any assumptions as to why they save everything, they just do, end of story.