Proceed with caution but not necessarily a scam.
Gumtree replies now go through gumtree somehow without revealing the various party's email.
If the seller agrees to exchange in person and has the original receipt then ok, continue with the transaction at your own discretion.
If its the usual, 'I'm actually in London/NY/Paris right now but can arrange for X/Y/Z watertight safe method of payment' then just report the ad.