I never understood why someone would buy a kit already put together. For me the fun is building it.
De gustibus non disputandem est. I wouldn't sell mine, either, for sentiment and for the hassle of posting the sale, dealing with collecting the payment, packing up and shipping. But some folks don't mind it. I wouldn't collect kits, either-and a stash doesn't count!-but some do.
As far as places to sell online are concerned, if not eBay, then perhaps through forums, or through other social media like Facebook, Etsy, etc. Personally, I can't speak for any of the social media because I haven't sold anything online, I've only bought, and apart from buying through an online shop, like Sprue Brothers, HobbyLink Japan, etc, etc, I've only bought through eBay (since '96). I did see something on Facebook's market page once-locally-collected honey-and bought it, but since the seller was local, we carried out the sale live, rather than through the webpage and with shipping.