I watched eBay for almost two years before jumping on one that came up cheap with a buy it now. During that time, every auction that ran its course achieved a four figure sum regardless of spec (bar one in a far flung location). I think to maximise exposure and price, eBay is by far the best option.
If you'd prefer not to pay the commission then gumtree is worth a shot if not in a hurry. Since eBay bought it, you get ads showing up across platforms if relevant to a search so might get picked up by anyone searching eBay.
Finally, there's a uk family cycling Facebook group where you'll find an audience of people interested in niche, child friendly bikes, including tandem and cargo bikes etc.
Finally, sell accessories separately, including child seats, non-circe specific racks etc will not as to the price but should get a little if sold separately. And sell in spring when everyone is planning for holidays and family bike rides
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