Rescuing animals from those kind of sites is a great thing! The animals listed are usually in pretty bad situations, unlike rabbits available at a rescue who are safe and well cared for.
I’ve rescued various animals and a few rabbits off of the US version of the site which is called Craigslist. Some were horribly neglected/abused. One thing you really need to keep in mind is the rabbits on Gumtree/Craigslist will have no health guarantee. I’ve spent hundreds of dollars in vet bills on animals rescued from Craigslist. You might need to spay and neuter the rabbit yourself. So the possibility of health problems or a vet bill is very real. Also, you do not know their temperament unless you meet the rabbit beforehand, unlike a rabbit rescue who will know each rabbit’s personality very well.
The ethical and moral stance is the same in my opinion, so long as you don’t buy the rabbit. Paying a small re-homing fee is fine since it’s recommended people not list rabbits as free to prevent bunnies from going as snake food. Try and make sure the person isn’t a breeder. Many like to use those kinds of sites even if they aren’t supposed to.