At least I think it's a scam - it certainly looks like one.
I advertised a cooker on the site a while ago, put my phone and email details on the ad. Later got a voicemail saying that someone was interested, please call this premium rate number to be put through to the buyer. If you call the number a recorded message witters at you for ages in order to run up the cost of the call, then says you will be put through. After half a dozen rings the line cuts off and a message says there was no answer.
On reflection I decided I'd been scammed, as my contact details were on the site for people to get in touch with me direct, and I see nothing on Leboncoin saying that buyers will be routed through any such system.
Looking at my phone bill, this has cost me the princely sum of 2,50€, which is more irritating than anything else but still the price of a beer and 2,50€ these tosswits weren't entitled to. Grr.
So ... if you get such a message, don't bother calling. And meanwhile, does anyone need a gas cooker? As new, 100€, Grenoble area. :-D