If I can put your mind at rest somewhat.
I have just rehomed an adult cat through Gumtree. She was one of two sisters but unfortunately one of the sisters was hit by a car just before she was due to come to me so we have the other sister now on her own. I went and visited the cats at their home, the owner then came to my house to see my house. We are in contact through Facebook. No one is trying to feed to cat to a Pitball.
Years ago due to a very bad change in my circumstances I had to rehome my adult cat through Gumtree. I am still in contact, my cat is very happy in his new home.
I have also in the past rehomed a cat through Wood Green. They did not visit my house, they gave me an indoor cat which in hindsight should have remained an indoor cat. I said I was going to let the cat out, they said that was absolutely fine and it would be good for him, a few weeks later he went out and never came home. I suspect he was either stolen (he was a breed) or he hurt himself and perished. I feel terribly guilty about it even now.
So charities do not always get it right and Gumtree is not populated by psychotic cat killers. Yes there are weirdos all over the place but in my experience of owners trying to rehome their adult cats, they adore them, are devastated they have to let then go, and try their absolute hardest to make sure they get rehomed in a fabulous new home.