I'm not sure about the used bike market across the pond, but after a few years of selling several via online classified in the US (mostly via Craigslist), I've found that the wording and photos I use in my ads have a LOT to do with the legitimacy or seriousness of the responses I get, as well as the eventual prices I fetch. This includes a couple I've sold since pandemania struck.
In my ads, I provide several decent photos from multiple angles (backgrounds matter!). And I don't simply provide a link to specs, I detail them, along with any customizations. Within the ad, I usually bury some codeword or phrase I ask folks to include in their response, preferably in the SU heading. I don't cut and paste from the mfr's marketing hype - most legit buyers will search that crap out themselves.
I also say something like, "Lowball offers and 'is it still available?' inquiries," especially those with no specific questions about the unit I'm selling, "will be ignored." Believe it or not, this helps weed out tire kickers, real-life and virtual.
Off-topic ( a bit), I've been taking notes on some of my buyers, particularly the ones who, like me, have health issues and need pedal assist to be able to ride again - it's inspiring. One guy, a retired airline pilot, drove 13 hours because he'd been searching that far to find the specific model I'd listed (Bulls E-Stream EVO FS3), and I didn't want to deal with HazMat shipping BS. I've actually made riding buddies out of a few buyers from in my region, and (pre-pandemic) would meet them at trailheads roughly halfway between us. One dude lives about a half hour away - we've probably done more than a dozen rides the past couple years.
I'm using some of their stories to help me write a biz plan for my 4th career change (at 58 YO - yikes!), and already have a few "references" attesting to folks' experiences buying ebikes from me. Ya never know where these things will lead!