I find any dealings on Gumtree that involve transferring of money or buying items based off photos or sight unseen is bad news, you are always taking a huge risk - especially with big ticket items like detectors or otherwise.
Too many people treat Gumtree as a poor man's version of Ebay (minus the fees), when it should be treated purely as a method of buying items face to face with the seller/buyer, and with cash in hand. That also includes meeting at a pre-arranged location to do the transaction, not at your home. There have too many cases of people having their home cased out by those with no intention of buying.
Personally I'd rather stick to ebay and cop the fees, whilst having some sort of protection. I've had nearly 700 transactions so far on ebay both in Oz and abroad, with only 2 bad dealings amongst those, both resulted in my funds being returned via paypal. If I receive an item like a detector doa, or not as described on Ebay, I can get my money back. Buy something similar on Gumtree that turns out be a boat anchor, there is not a hell of a lot you can do about it.
I saw this warning on a forum re scamming on gumtree, all so very true:
if they're not in the locality and express interest, then beware;
if they cannot provide a local contact at all, be very suspicious;
if they are prepared to purchase sight unseen, be very suspicious;
if they provide a story that just doesn't feel right or make sense, they're scammers;
if they make too-good-to-be-true promises, they're scammers;
if they seek personal and especially financial details; they're scammers.