GasGuzzler has a good point. The guy who adds stuff,, modifies stuff, etc then expects others to want what he likes is often very disappointed when it doesn't sell.
I had a good friend, and older gentleman who loved to "tinker & modify" most of his guns. I could have cried at what I saw him do to a few very desirable guns. But I NEVER said a word,, as they were his guns, and his idea as to what he wanted.
He gave me a NM .45 Blackhawk long ago. The ugly, plastic grips (sorta grips) were epoxied on the g/f. And a few other things had been done. Now,, I truly cherish the gun, because it's a Ruger, and a gift from a friend. But I had to try & return it to as close to original as I could.
When he made the decision to sell off his guns & such,, he asked me to assist him. I did,, and he didn't get as much for his guns as he could have if they had been left alone. His Colt 1911 Gold Cup Match was so "modified" that it only brought $400. He wasn't stupid,, and he KNEW he'd removed any real value for most of his guns. But he told me; " I wanted them to be used by me in my way!" And since his family were all basically anti-gun, and such, they didn't care either.
I guess the worst thing to me is when a person has a rarer variation of a gun,, and wants to modify it beyond it being returned to "as shipped" condition, refuses to sell that gun, and buy one like it that's much more common. I saw a OM Ruger FT "Ross" variation once that had been made into a .44 Spl. There is only about 800 of the "Ross" guns. Sell it at a premium, and buy a more common used shooter FT & customize that.
Of course,, they aren't my guns to make that kind of decision. But you can bet that any of my guns that might be collectable,, won't get mods that aren't reversible.