Gumtree you meet face to face. Why didn't your friend check the phone there and then?
Also surely you have the seller's mobile number so that you could meet up. If people are ignoring you on their mobile then something is up.
Never buy phones on gumtree as there is a massive risk. No reputation system means they could be regular scammers. Even if you've physically checked the phone properly when buying, they could simply report it stolen afterwards to also get an insurance payout.
Obviously there are honest people, but as a buyer I would stick to ebay on phones.