Just underneath the photo on each gumtree listing there is a "Report Ad" link [at least that's where it is at the moment...they'll probably move it again when they make yet another of their constant cosmetic tweaks
].
Gumtree tend to be rather ineffective in addressing concerns though. I've reported pretty obvious dodginess and listings that break their own posting rules before without any result. Also notified them of problems with their site with mixed results. Usually get a standard proforma response to those suggesting that it's probably at my end [standard IT Crowd "have you tried turning it off and on again" kinda thing]. A few things have been attended to in a week or so, others not at all....probably working on an ease of remedy system rather than one that considers importance.
Since they seem quite lax about things, I suspect you'll need to be able to offer them something quite substantial to get them to act on listings. Just annoying them with hunches may result in even less attention being paid to definite dodginess in future [see the boy who cried mongoose...sorry, closest my mind could get
]. And taking it up with any higher authority would definitely require some rather substantial evidence...you've seen through this forum how hard it is to get them to move, even if it is handed to them on a plate.
I was thinking about your initial post again during a ride the other day. The listings that tend to pique my cautionary reflexes are most likely to be from sellers who have only recently set up an account, are only selling a single item, and tend to have poor descriptions and photos. Those that list multiple items regularly, and have done so for some time, are more likely to fall into that category I mentioned earlier. Also considered transporting goods across country. I guess if stock requires zero outlay it would be economically feasible, but again, selling in such an obvious venue seems awfully risky. I know it's one of the first places I'd be keeping an eye on if I ever lose another bike. Also, if all the crime shows I watch are anything to go by, the sort of people who tend to nick things seem to be quite keen to convert them to cash [or other erm, "goods" if they have a regular buyer]. If anyone in that line of business were going to be listing hot items so publicly, I suspect they'd be trying to offload as quickly as possible and therefore unlikely to be trying their luck at over the odds prices.
On Chris's flipping front - I'm vaguely aware of the ruckus at least one of the dodgy gumtree seller's that came to BNA's attention caused. By all means, go after sellers that are obviously misleading. But overpricing alone is unlikely to be anything gumtree, ebay, or elsewhere are going to worry about. Their sites are chock full of listings asking ridiculous prices for all manner of things[1]. It's the nature of the beast I guess...many sellers tend to think their stuff is worth a small fortune whereas most buyers tend to be quite reluctant about handing over their hard earned. And there's the rub - buyers are expected to attend to their own budgetary requirements. You can try and hold their hands all you like if you have the time, but the type of folk who buy on impulse without much consideration will continue to do so despite your efforts, and that being the case, people will always be willing to try and exploit it. Good on you for trying but I think you might be attempting to push fresh poo up a 20% gradient with a cable end.
Or to look at it another way - it's pretty much how a free market works, isn't it? Folk generally try to sell for what they think they can get away with and buyers are free to look elsewhere if they don't like it. If there's a gap [or perhaps even if there isn't] some middleman will eventually fill it and try to claim their slice of the pie. Sometimes, and I suspect that this might occur with both the guitar and bike trades in some instances, manufacturers work with middlemen [otherwise known as importers] to screw every last cent out of your local shop and therefore the end user. Under other systems you might be entitled to one bike that looks exactly like everyone elses. That might solve the theft and overreachingaskingprices problems, but how many folk here would be willing to hand in all their lovely bikes to do so?
Apologies for another long one. I have to think of something whilst turning the pedals.
[1]eBay often makes determining a going rate for popular items much easier since they allow you to see recent sales. Try picking a few items to see what they've gone for and then look at the wide range of current asking prices for same. The whole system might work a little better if both sides did a modicum of research....buyers would be less likely to pay over the odds and that would force sellers to be more reasonable if they really wished to sell. Then again, if it's still happening on an almost universally accessible buying/selling site with that feature, then perhaps saving everyone from themselves is a goal that's unlikely to be achieved?