You covered everything I was going to say
What do you know about this bird's history, age, etc. And importantly, why does he want to rehome his goffin? My goffin is also wild caught. I don't know much about his history other than he's been through multiple homes before mine. Goffins are crazy smart. Almost to the point it's scary. My goffin is busy all the time. ALL THE TIME. He destroys at least a toy a day. (It becomes a pretty big expense.) He too can and has escaped from his cage. He loves to fly and play and will literally fall over from exhaustion before he will willing go to bed. He also isn't food motivated but responds to attention and praise. I have found that keeping him on a schedule has greatly diminished the screaming. He knows when he's going to get out of his cage and when he's not. I would love to be more spontaneous with him but it results in screaming when I'm not home which drives my fiancé crazy. He is pretty independent but I too think that has something to do with him being wild caught. He'll do about 10 minutes of head scratches a day, if I'm lucky. The rest of the time he's too busy. But he is sweet, and funny, and playful and smart. But also destructive and really can't be left unsupervised at all. (I have chunks taken out of door frames and cupboards as a result of lack of supervision- like a couple minutes of lack of supervision). Also, I currently have 2 cockatiels in my house. I don't let them out with my goffin. I don't think he would deliberately hurt them but the size difference scares me. I love my goffin and think he's an incredible bird and certainly don't want to paint goffins in a bad light but just want people to have an understanding of what they're getting into. (And I do think females are a bit more laid back then the males. Is this bird a male or female?)He says the bird is a male and about 20 years old. He doesnt know a ton of history so I am assuming he has had a few homes already. He is currently being feed only seeds and fruits. He is also cage territorial but I'm told once he comes out on his own is a sweetheart. At this point it's all I really know.I agree with @MommyBird .
Well, although fruit is better limited, at least he's not eating all seeds. If he's eating fruit, my guess is it should be pretty easy to get him to eat vegetables too. Even if at first you make "chop" mixing fruits and vegetables. I think most birds are cage territorial. I always let my cockatoo come out on his own. And 99.9% of the time he wants to come out, so that's not an issue. He is completely fine with my hands in his cage but he doesn't like other people's hands in in his cage. I think that partially is a trust issue. But cage's with feeder doors eliminate a lot of that problem. The good thing about his age is he's well past puberty. So you wouldn't be dealing with that issue. So 20 is probably about middle age-ish for a goffin. So you're likely looking at least another 20 years with him. I would meet him and see what you think.
Sadly it's worse as she gets older. 
Taking in this bird is going to be a huge challenge no matter what stories or experiences are given. You have some small bird experience so you won't be flying totally blind but this is a whole new level of bird ownership. (If you have a child you have a pretty good comprehension of the commitment because it's on that level).
WIth a Goffin , if you buy one of the bird kabobs available at many vendors and hang in cage with raw veges on it (corn rounds, carrots, pea pods, peppers, broccoli, etc) he will at least shred it up and quite possibly learn to eat some in the process. They can't resist something that needs to be taken apart.
