Well, Gumtree is nothing more than a classified ad medium, an internet version of the local paper or Loot. Use the same common sense that you'd use replying to a printed classified ad.
I think it's fairly easy to judge is something is genuine or not. I enquired about a bike I saw on Gumtree, seemed a bargain. When I phoned up about it, the seller said he was away, and the bike was no longer at his home but at a mates place. Could have been genuine, but it put me off and I left it.
If someones selling stolen goods they won't want you to know where they live. Pretty handy if you don't know what they look like either and just have a name and mobile number to go on!
So, I wouldn't avoid Gumtree ads, just be cautious.