Ouch.
That was a more elaborate scam but my sister & I learnt more than a decade ago (on ebay, when bank deposit was the only way someone could pay) not to trust what people say, but to wait till money actually showed in the bank account.
I don't know how many times we had buyers swear black & blue they had paid, but my sister would always reply-sorry, it is not showing up in the bank, you'll need to go to your bank to check what has happened etc
Each & every time, it turned out they had not paid and she would get a reply along the lines of 'Okay, the money will be sent tomorrow".
I know the man in the article saw what he thought were legit documents such as drivers licence & payment receipts etc but the same rule should apply-no one picks up the goods till the money shows up in his bank account. If he had messaged that to note to the seller that to be clear, no pick up could happen till funds had cleared, he would have been fine.
Gumtree is a funny sort of site. You get some time wasters who say they'll come to look at items but don't.
I've personally found that small items on gumtree don't sell well, though if you go on facebook local buy swap sells etc they may.
Gumtree is very quiet. I recently sold some as new carpet runners. I had 18 queries on facebook within 24 hours and they were all picked up next day. One query only on gumtree and person then never got back to me. It makes me wonder whether gumtree will remain or go under as it seems very sluggish.
